Refactor split migrator

LMS-2936

Removed a bunch of loc_mapper code
Cleaned up offering now being course and run in a few places
Removed extra db queries

Conflicts:
	common/lib/xmodule/xmodule/modulestore/tests/test_publish.py
damn, bad change. I got confused about the scope of the else and thought it was a 3rd scenario not the default one. Does anyone find the else helpful v just putting the line at same level as the try?