Remove LocMapperStore

LMS-2918

@nasthagiri @sarina plz review
Actually, I need to have it explicitly support both old mongo and xml in the "does this course exist" test. It's an interesting problem b/c the ones in loc mapper must have been mongo backed and not xml; thus, for decoding the dotted forms just checking mongo suffices. For checking the abridged forms (e.g., instructor_myfavorite.course), it must check both stores. I'll double check it.