Revert multiple cohorts feature and add a revert management command

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. 

This PR is the complete revert + the addition of a management command to ensure things are ok. To use the management command, run:

```
./manage.py lms revert_workgroup 
```

You should see a output of the current issues and some instruction to fix them:

```
To fix issues:
1. Ensure auto_cohort is enabled and that there is a default cohort in auto_cohort_groups for all courses.
2. Run this script with the "--fix" option.
3. You have to fix the *in multiple cohorts" errors manually using the LMS instructor dashboard.
```

Then, run the management command with the --fix option and apply the manual fixes if there are.