CLN: _convert_scalar_indexer only handle "loc" and "getitem"
`_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.