MAINT: Remove self.assertNotEqual from testing
Title is self-explanatory.

Partially addresses #15990.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=h1) Report
> Merging [#16176](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/de8734474daddf772d97c66a9ef759e23a3d362f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16176/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16176      +/-   ##
==========================================
- Coverage   90.88%   90.86%   -0.03%     
==========================================
  Files         162      162              
  Lines       50821    50821              
==========================================
- Hits        46189    46176      -13     
- Misses       4632     4645      +13
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.33% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.54% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16176?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/16176?src=pr&el=footer). Last update [de87344...549c3d7](https://codecov.io/gh/pandas-dev/pandas/pull/16176?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!