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 We can't ask to do things manually in the LMS, users/admins never connect there. It has to be done programmatically from the API code. The best way to ensure that it will work in all cases would be imho to do it the way I specified it in the task - by never letting the autocohorting feature kick in, and always ensuring that we explicitly place the users in one, and only one cohort. Also take into account the fact that some users will be in no cohort at all when we push this (since that's the current behavior), and that courses can be switched from non cohorted to cohorted, and that we need to account for that change too.