Deprecate partial slicing of unordered DatetimeIndex when both keys are not present
- [x] xref #18531
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

cc @jreback @jorisvandenbossche 

This would deprecate #37796 before removing it. 