CLN: resolve UserWarning in `pandas/plotting/_matplotlib/core.py` #35945
- [x] closes #35945
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

> thanks for the PR, @VirosaLi
> 
> just curious where is this warning getting caught or is the warning raised on a newer matplotlib version, do we have any tests right now to catch the warning?

pandas/tests/plotting/test_frame.py: 10 warnings
pandas/tests/plotting/test_series.py: 5 warnings
  /home/fangchenli/Workspace/pandas-VirosaLi/pandas/plotting/_matplotlib/core.py:1235: UserWarning: FixedFormatter should only be used together with FixedLocator
    ax.set_xticklabels(xticklabels)
 @VirosaLi so the warning is now gone? I don't think we actually need a test for this, but if you can put an assert_produces_warning(None) around something and have it pass would be great. the first one will be resolved once https://github.com/pandas-dev/pandas/pull/36085 gets merged. thanks @VirosaLi  @meeseeksdev backport 1.1.x Can't Dooooo.... It seem like this is already backported (commit is empty).I won't do anything. MrMeeseeks out.