Revert "BUG: fix union_indexes not supporting sort=False for Index subclasses"
Reverts pandas-dev/pandas#35098

Closes https://github.com/pandas-dev/pandas/issues/35238 I'll also push a test from https://github.com/pandas-dev/pandas/issues/35238 here, and add the xfailing tests from #35098.
thanks @TomAugspurger  Tests still need to be added?