BUG: DataFrame.drop raising TypeError for string label with non-unique MultiIndex and no level provided
- [x] closes #36293
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

As far as I know, isin needs a level input when only a string is given for a MultiIndex. If this should not change, we have to set level=0 in this case.