REF: define _convert_to_indexer in Loc
_convert_to_indexer is overriden by iloc, so the version currently defined in LocationIndexer is only used in Loc.  This moves it directly to Loc.

Also, since ix has been removed, we have an unnecessary level of the class hierarchy, so this merges NDFrameIndexer and LocationIndexer.

Follow-ups will move base class methods that are actually Loc-specific to Loc. 