Remove dead code for pyarrow < 15.0.0

Remove dead code for pyarrow < 15.0.0.

Code is dead since the merge of:
- #6892

Fix #7022.

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7023). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005669 / 0.011353 (-0.005684) | 0.004233 / 0.011008 (-0.006775) | 0.063550 / 0.038508 (0.025041) | 0.031269 / 0.023109 (0.008160) | 0.234280 / 0.275898 (-0.041618) | 0.264517 / 0.323480 (-0.058963) | 0.003310 / 0.007986 (-0.004676) | 0.003640 / 0.004328 (-0.000688) | 0.050139 / 0.004250 (0.045889) | 0.046909 / 0.037052 (0.009856) | 0.253101 / 0.258489 (-0.005388) | 0.280281 / 0.293841 (-0.013560) | 0.029558 / 0.128546 (-0.098989) | 0.012537 / 0.075646 (-0.063110) | 0.209624 / 0.419271 (-0.209648) | 0.036857 / 0.043533 (-0.006676) | 0.236957 / 0.255139 (-0.018182) | 0.260510 / 0.283200 (-0.022689) | 0.019802 / 0.141683 (-0.121881) | 1.141747 / 1.452155 (-0.310407) | 1.172617 / 1.492716 (-0.320099) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.107381 / 0.018006 (0.089375) | 0.308401 / 0.000490 (0.307911) | 0.000227 / 0.000200 (0.000027) | 0.000056 / 0.000054 (0.000001) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.019504 / 0.037411 (-0.017907) | 0.063920 / 0.014526 (0.049394) | 0.075375 / 0.176557 (-0.101181) | 0.122707 / 0.737135 (-0.614428) | 0.080015 / 0.296338 (-0.216324) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.288716 / 0.215209 (0.073507) | 2.862022 / 2.077655 (0.784368) | 1.472510 / 1.504120 (-0.031610) | 1.332989 / 1.541195 (-0.208206) | 1.395140 / 1.468490 (-0.073350) | 0.728042 / 4.584777 (-3.856735) | 2.409914 / 3.745712 (-1.335799) | 2.912514 / 5.269862 (-2.357347) | 1.986980 / 4.565676 (-2.578697) | 0.078587 / 0.424275 (-0.345688) | 0.005601 / 0.007607 (-0.002006) | 0.342510 / 0.226044 (0.116466) | 3.354621 / 2.268929 (1.085692) | 1.852472 / 55.444624 (-53.592153) | 1.542567 / 6.876477 (-5.333910) | 1.726756 / 2.142072 (-0.415317) | 0.794567 / 4.805227 (-4.010660) | 0.135279 / 6.500664 (-6.365386) | 0.042591 / 0.075469 (-0.032878) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.968336 / 1.841788 (-0.873452) | 12.334614 / 8.074308 (4.260305) | 9.638775 / 10.191392 (-0.552617) | 0.143625 / 0.680424 (-0.536799) | 0.015475 / 0.534201 (-0.518726) | 0.313357 / 0.579283 (-0.265926) | 0.271257 / 0.434364 (-0.163107) | 0.362074 / 0.540337 (-0.178263) | 0.468595 / 1.386936 (-0.918341) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.006243 / 0.011353 (-0.005110) | 0.004496 / 0.011008 (-0.006512) | 0.051271 / 0.038508 (0.012763) | 0.035718 / 0.023109 (0.012609) | 0.272623 / 0.275898 (-0.003275) | 0.297060 / 0.323480 (-0.026420) | 0.004801 / 0.007986 (-0.003185) | 0.003060 / 0.004328 (-0.001269) | 0.049990 / 0.004250 (0.045740) | 0.042413 / 0.037052 (0.005360) | 0.281268 / 0.258489 (0.022779) | 0.327224 / 0.293841 (0.033383) | 0.033745 / 0.128546 (-0.094801) | 0.012777 / 0.075646 (-0.062869) | 0.061808 / 0.419271 (-0.357464) | 0.034428 / 0.043533 (-0.009105) | 0.272211 / 0.255139 (0.017072) | 0.327260 / 0.283200 (0.044061) | 0.019756 / 0.141683 (-0.121927) | 1.137768 / 1.452155 (-0.314387) | 1.220347 / 1.492716 (-0.272369) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.099737 / 0.018006 (0.081731) | 0.304627 / 0.000490 (0.304137) | 0.000210 / 0.000200 (0.000011) | 0.000052 / 0.000054 (-0.000002) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.023177 / 0.037411 (-0.014234) | 0.077505 / 0.014526 (0.062979) | 0.088957 / 0.176557 (-0.087599) | 0.129187 / 0.737135 (-0.607948) | 0.090386 / 0.296338 (-0.205953) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.291558 / 0.215209 (0.076349) | 2.874297 / 2.077655 (0.796642) | 1.562316 / 1.504120 (0.058196) | 1.439950 / 1.541195 (-0.101244) | 1.492316 / 1.468490 (0.023826) | 0.729885 / 4.584777 (-3.854892) | 0.985075 / 3.745712 (-2.760637) | 3.108313 / 5.269862 (-2.161549) | 1.998072 / 4.565676 (-2.567604) | 0.079367 / 0.424275 (-0.344908) | 0.005210 / 0.007607 (-0.002398) | 0.347335 / 0.226044 (0.121290) | 3.519375 / 2.268929 (1.250446) | 1.949395 / 55.444624 (-53.495229) | 1.650379 / 6.876477 (-5.226097) | 1.691606 / 2.142072 (-0.450466) | 0.816023 / 4.805227 (-3.989204) | 0.135318 / 6.500664 (-6.365346) | 0.041390 / 0.075469 (-0.034079) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.018964 / 1.841788 (-0.822823) | 13.120135 / 8.074308 (5.045827) | 10.618095 / 10.191392 (0.426703) | 0.134507 / 0.680424 (-0.545917) | 0.015895 / 0.534201 (-0.518306) | 0.302864 / 0.579283 (-0.276420) | 0.131117 / 0.434364 (-0.303247) | 0.342374 / 0.540337 (-0.197964) | 0.441640 / 1.386936 (-0.945296) |

</details>
</details>

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