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. 

well, the autocohorting change (about forcing a default_cohort if there is none in auto_cohort_groups) is not yet committed. Isn't in their PR you need to see the validation/behavior if a course switch from non-cohorted to cohorted?

I'm not sure to know how & where I should add the stuff you want to be done. I understand what you described, but I don't know how to ensure all that stuff in all the parts of the application. There is user registration, student list upload and I'm not aware of everything in the client app. There is nothing done to select a cohort on creation IIRC. Do you have a more concrete plan for what you have in mind? Also, why can't we use the autocohort by default? What are the side effects of this? They will be move to the proper cohort when the teacher know its workgroup, no?

CC @martynjames @mattdrayer if they have useful hints to give me