CLN: _convert_scalar_indexer only handle "loc" and "getitem"

[jbrockmendel]

_convert_scalar_indexer is called with kind="iloc" from only one place, and in that case

1. the base class method is equivalent to just the 1-liner self._validate_indexer("positional", key, "iloc")
2. all subclasses just call the base class method

So by inlining that 1-liner, we can take the "iloc" case out of _convert_scalar_indexer altogether.

kind=None is never passed, so we can rip that right out.

Ultimately I want to disentable/de-duplicate/disambiguate _convert_scalar_indexer vs _maybe_cast_indexer

Partial overlap with #31625.

[jreback]

can you rebase

[jbrockmendel]

rebased+green