BUG: DataFrame.drop() does nothing for non-unique Datetime MultiIndex
- [x] closes #12701
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Closes #12701

Follows the suggested fix in the comments to the bug report.
Also added a line in whatsnew and a test.

Regarding the tests it passes the one I added. But I just noticed that plenty others fail and I have yet to find out whether it's caused by this (who knows, it might)
Update: It's the same tests failing on master branch as on this one (phew..). I guess the errors deserve bug reports but that's for another time.

@jonaslb nothing fails on master main build. There is a single failure on 2 of the compat builds, but these are known.

are you talking about something specific?
 @jreback [Here's a gist](https://gist.github.com/jonaslb/b8ad05c4ed04b11fe9cfe633e09ca070) (7 errors / 2 failures) of my test-run. Yes I guess it's specific since it's something with clipboard and then there are some index and slice problems apparently. But lets wait and see what travis says - I'm pretty certain I didn't mess it up with this commit :) Sometime later I'll look more into it and maybe make bug reports.
 thanks @jonaslb 
