Revert multiple cohorts feature

Following the discussion on the upstream PR: https://github.com/edx/edx-platform/pull/4364 , it looks like this feature is not needed anymore for the client need. 

@aboudreault With apros the users are always created and assigned to courses via the API - so you don't need to care too much about the other ways to register for a course.

About the autocohorting, it comes from the fact that we have to make it work both with and without the autocohorting feature, since this is going to arrive only later on - I don't want to have to refactor this again when it lands. Also, ensuring we cover all the cases from the API without relying on magic behavior is likely to be less brittle to further changes to the default cohorting behavior down the line.