CLN: remove doc/plots directory (dead code)

[topper-123]

• 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 I assume the dir existed and had uses from the time before @savefig could be used in ipython directives, and now is outdated.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]

Codecov Report

Merging #18751 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            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)	⬆️
#single	40.74% <ø> (-0.12%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️

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[jreback]

thanks!