REF: clear out a bunch of algos, de-duplicate a bunch of core.missing
State: closed

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Labels:
name: Missing-data
description: np.nan, pd.NaT, pd.NA, dropna, isnull, interpolate
name: Clean

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=h1) Report
> Merging [#24652](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/393d0aa1dc1b2b0cd478cbded6fb13e530589c4e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24652/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24652      +/-   ##
==========================================
+ Coverage   92.37%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52384    52317      -67     
==========================================
- Hits        48390    48330      -60     
+ Misses       3994     3987       -7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <100%> (-0.01%)` | :arrow_down: |
| #single | `43.06% <27.77%> (+0.03%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24652/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `92.56% <100%> (+0.26%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24652/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.09% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=footer). Last update [393d0aa...e42bc27](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=h1) Report
> Merging [#24652](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/393d0aa1dc1b2b0cd478cbded6fb13e530589c4e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24652/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24652      +/-   ##
==========================================
+ Coverage   92.37%   92.37%   +<.01%     
==========================================
  Files         166      166              
  Lines       52384    52317      -67     
==========================================
- Hits        48390    48330      -60     
+ Misses       3994     3987       -7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <100%> (-0.01%)` | :arrow_down: |
| #single | `43.06% <27.77%> (+0.03%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24652/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `92.56% <100%> (+0.26%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24652/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.09% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=footer). Last update [393d0aa...e42bc27](https://codecov.io/gh/pandas-dev/pandas/pull/24652?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

@jbrockmendel happy to merge
everyone happy here?
lgtm