DEPR: Remove (Int|UInt|Float)64Index
Actually removes `Int64Index`, `UInt64Index` & `Float64Index`. Also does some cleanups that previous deprecation PRs didn't catch.  

This PR formally achieves the goals set out in #42717. In real terms there are a few outstanding items, which still need to be addressed:
* Move `NumericIndex` into `Index`
* Update docs and write the `whatsnew` entry for this change
* Make the tests in `pandas/tests/arithmetic` cover 8/16/32 bit dtypes
* a few minor issues

So I'd like to kep #42717 open until those are resolved.
 
The remaining failure look unrelated. Thanks @topper-123 