[tests] remove deprecated tests for model loading
Author: faaany
Pull request: yes

## What does this PR do?
2 tests keep failing in `test_modeling_utils.py`:
```bash
FAILED tests/test_modeling_utils.py::ModelUtilsTest::test_legacy_load_from_url - huggingface_hub.utils._validators.HFValidationError: Repo id must be in the form 'repo_name' or 'namespace/repo_name': 'https://huggingf...
FAILED tests/test_modeling_utils.py::ModelUtilsTest::test_load_from_one_file - huggingface_hub.utils._validators.HFValidationError: Repo id must be in the form 'repo_name' or 'namespace/repo_name': '/tmp/tmpoa14so_e...
```
The reason is that `https://huggingface.co/hf-internal-testing/tiny-random-bert/resolve/main/pytorch_model.bin` and `/tmp/xxx` are not valid `repo_id`s. Currently, only `path to a local folder` or `repo_id of a model on the Hub` are accepted. 

Below is the detailed error message:
```bash
E           huggingface_hub.utils._validators.HFValidationError: Repo id must be in the form 'repo_name' or 'namespace/repo_name': 'https://huggingface.co/hf-internal-testing/tiny-random-bert/resolve/main/pytorch_model.bin'. Use `repo_type` argument if needed.
/home/fanli/.conda/envs/dev/lib/python3.10/site-packages/huggingface_hub/utils/_validators.py:158: HFValidationError
```

In my opinion, this is technical debt and we should only test up-to-date features. So I remove them instead of modifying them. What do you think? @ArthurZucker @ydshieh


Comments:
Hi @ydshieh, could you help review this PR? In my opinion, these 2 tests are just technical debts and should be cleaned-up.

Hi @faaany Thank you for flagging this situation and opening a PR. I am up to remove these 2 tests, but on our CI history, I saw they started to fail on August 18 2023. However, I can't see any change in `huggingface_hub` at/around that day.

So I would like to ping @Wauplin to see if he remembered something during that time period. 

@ydshieh can't tell what changed on August 12th 2023 specifically on this. The `huggingface_hub.utils._validators.HFValidationError` is expected if an incorrect repo_id is passed (typically a URL or a path) to any `huggingface_hub` method. But this validator dates back to August/Sept 2022 so quite a long time before. I would say most likely how things are handled internally in `transformers` have changed and a call to `huggingface_hub` is now made without verifying if it's a path/url or not.

I'm fine with removing completely the tests but then it means the feature must be considered as removed from `transformers`, not just deprecated until v5. For the end user it doesn't change anything since the feature is broken anyway. (by feature I mean "load from single file or URL"). => it would be good to check if there is any mention to this or any logic in `transformers` related to this feature and remove them altogether so that we don't keep dead code in the main source code. 

Thank you @Wauplin for the comments. Good points.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_29450). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@amyeroberts @Wauplin 

Got to say I never knew this feature (loading from url or file) exists. Quick search I found nothing extra to be removed, but I don't get better idea what to check.

If you have any idea, let me know. Or we can ping Lysandre. Otherwise if you feel ok, we can merge anyway.

@amyeroberts thanks for the review! I removed all equivalent tests from test files that have the function `test_legacy_load_from_url` and the comment `This test is for deprecated behavior and can be removed in v5`. Pls help double-check. Thx! 

> Awesome - thanks for this tidy up!
> 
> Could you try rebasing on main? I think this should resolve the currently failing (unrelated) test

done!