Deprecate inplace in Categorical.remove_unused_categories
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Related to #37643

This is a very good start.  For the checkbox at the top, since this doesn't handle all the relevant methods, it doesn't close the issue.  Instead write below the checkboxes "xref #37643" @jbrockmendel Thanks for a quick review, I have updated this pull request according to your suggestions Hello @OlehKSS! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-11-18 11:57:24 UTC thanks @OlehKSS happy to have you do the rest ....