CLN: remove ABCIndex
It's a footgun.

BTW I expect the CI to fail until #38020 goes in, which will [demonstrate](https://github.com/pandas-dev/pandas/pull/38020#discussion_r529159757) that there is a test case that uses MultiIndex.
need a rebase.

should consider renaming ABCIndexClass -> ABCIndex lgtm ping on green. ping nice. also helps for consistency when casting for mypy
 > should consider renaming ABCIndexClass -> ABCIndex

+1