ENH: Implement subtraction for object-dtype Index
- [x] closes #19369
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@jreback @jorisvandenbossche discussed briefly at the sprint.  Merits more thorough testing, but I'd like to get the go-ahead to separate out arithmetic tests that are common to EA/Index/Series/Frame[1col] that are highly duplicative first.