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
@nasthagiri what should I be using in place of `ModuleStoreEnum.BranchName.published` as the branch id in split? I can't use `MongoRevisionKey.published` b/c it's None. This is the key that gets saved w/ the data.