Deleting a group can lead to orphaned group forum discussions

To reproduce

1. Having a course with groups A and B, create a forum activity with its Groups mode set to "Separate groups"
2. Create a discussion for Group B.
3. Delete Group B
4. Check the forum_discussions table.
5. You'll see that the discussion for Group B still contains group B's ID in the "groupid" column.

This orphaned discussion won't be rendered in the UI anymore. So we need to do something about this. Either:

1. We delete the group discussion; or
2. Remove the group ID, exposing the forum discussion to all the course participants.

Other suggestions/ideas welcome.