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. 

@antoviaque , Apparently, it doesn't look like we can add this validation in the model since the ManyToManyField is not accessible when creating the object (since it is not saved yet). So django simply crash saying that we can't use the workgroup.users. i guess this would require a add_users_to_workgroup function (like the cohort API) to do that external check. (which is what we do in the views.py). But this wouldn't solve the main issue since it is not a validation in the model itself.