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
The `location` variable is still used in the rest of this file, but no longer initialized.

I guess it doesn't matter at this time, since this code seems outdated - must not be called anywhere.