CLN: remove doc/plots directory (dead code)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

As part of my work on #18723, I've stumbled upon the ``doc/plots`` directory. I've searched for usages of files in this directory and have turned up blanks only, so this whole directory seems to be dead code. I assume the dir existed and had uses from the time before ``@savefig`` could be used in ipython directives, and now is outdated.

As the pandas docs now relies on ``@savefig`` decorators, I propose just removing this directory.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18751?src=pr&el=h1) Report
> Merging [#18751](https://codecov.io/gh/pandas-dev/pandas/pull/18751?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d2fd22e1f995a638f1e84b0ef091eaf70e171fdc?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18751/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18751?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18751      +/-   ##
==========================================
- Coverage   91.59%   91.57%   -0.02%     
==========================================
  Files         153      153              
  Lines       51364    51364              
==========================================
- Hits        47046    47037       -9     
- Misses       4318     4327       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.44% <ø> (ø)` | :arrow_up: |
| #single | `40.74% <ø> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18751?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18751/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18751/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18751?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/18751?src=pr&el=footer). Last update [d2fd22e...abaf232](https://codecov.io/gh/pandas-dev/pandas/pull/18751?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!