API: Remove CalendarDay
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Given that we want to ship 0.24.0 soon and that converting `'D'` and `Day` to always act as calendar day warrants more game-planning, this PR just simply removes `CalendarDay` and reverts `'D'` and `Day` to their prior behavior. 