Course admin should be able to de-admin dicussion admins

As a global staff or course staff once you added yourself
in the discussion admins you cannot revoke it back unless
you remove yourself as course staff. Any of the course staff
member should be able to remove others from discussion admins
whether they are course staff or not.

TNL-315
Merge pull request #5363 from mlkwaqas/waqas/tnl315-cannot-remove-discussion-admin

Course admin should be able to de-admin dicussion admins