Remove ix, part 1
- [ ] xref #15113 and #26438 

First part of removal od ``NDFrame.ix``.  There's still alot of tests for this, so I'll do this removal in 2-3 steps. This PR removes pandas/tests/indexing/test_ix.py and tests for ix in a few more places.
Just seen #6581, where it says ix is to be removed in 1.2. So this is perhaps too early? Anything deprecated prior to 0.25 is good to remove for 1.0, so no issue there.

There is however #27620 out there as well ix is good to remove now I’ll just close this down, given that #27620 is already on this...