REF: remove iloc case from _convert_slice_indexer
_convert_slice_indexer is turning out to be one of the stickier methods to figure out, xref #31658.

The case with kind=None is only called from one place in core.indexing, and that is not reached in the tests.  I'd like to either a) figure out a non-None kind to pass to it, or b) determine that it can never be reached so can be removed.  Any thoughts on this are very welcome.