This commit removes code that was used to copy Old Mongo courses into
new Split Mongo courses. This includes both the migrate_to_split
management command, as well as the backend code that would be invoked
to re-run Old Mongo courses as Split courses using Studio (the UI for
this was already removed in b429e55c).
This is a part of the Old Mongo removal effort tracked in:
https://github.com/openedx/public-engineering/issues/62