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assume the dir existed and had uses from the time before ``@savefig`` could be used in ipython directives, and now is outdated.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