Remove code related to Old -> Split Migration

The [migrate_to_split](https://github.com/openedx/edx-platform/blob/master/cms/djangoapps/contentstore/management/commands/migrate_to_split.py) management command and its supporting code are no longer relevant, and should be removed. That being said, we need to be careful to _not_ remove the ability to re-run courses in general, as that logic may be mixed in there.