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

lms/djangoapps/instructor/tests/test_api.py

@@ -1299,35 +1299,22 @@ class TestInstructorAPILevelsAccess(ModuleStoreTestCase, LoginEnrollmentTestCase
         # Seed forum roles for course.
         seed_permissions_roles(self.course.id)
 
-        # Test add discussion admin with email.
-        self.assert_update_forum_role_membership(self.other_user.email, "Administrator", "allow")
+        for user in [self.instructor, self.other_user]:
+            for identifier_attr in [user.email, user.username]:
+                for rolename in ["Administrator", "Moderator", "Community TA"]:
+                    for action in ["allow", "revoke"]:
+                        self.assert_update_forum_role_membership(user, identifier_attr, rolename, action)
 
-        # Test revoke discussion admin with email.
-        self.assert_update_forum_role_membership(self.other_user.email, "Administrator", "revoke")
-
-        # Test add discussion moderator with username.
-        self.assert_update_forum_role_membership(self.other_user.username, "Moderator", "allow")
-
-        # Test revoke discussion moderator with username.
-        self.assert_update_forum_role_membership(self.other_user.username, "Moderator", "revoke")
-
-        # Test add discussion community TA with email.
-        self.assert_update_forum_role_membership(self.other_user.email, "Community TA", "allow")
-
-        # Test revoke discussion community TA with username.
-        self.assert_update_forum_role_membership(self.other_user.username, "Community TA", "revoke")
-
-    def assert_update_forum_role_membership(self, unique_student_identifier, rolename, action):
+    def assert_update_forum_role_membership(self, current_user, identifier, rolename, action):
         """
         Test update forum role membership.
         Get unique_student_identifier, rolename and action and update forum role.
         """
-
         url = reverse('update_forum_role_membership', kwargs={'course_id': self.course.id.to_deprecated_string()})
         response = self.client.get(
             url,
             {
-                'unique_student_identifier': unique_student_identifier,
+                'unique_student_identifier': identifier,
                 'rolename': rolename,
                 'action': action,
             }
@@ -1336,7 +1323,7 @@ class TestInstructorAPILevelsAccess(ModuleStoreTestCase, LoginEnrollmentTestCase
         # Status code should be 200.
         self.assertEqual(response.status_code, 200)
 
-        user_roles = self.other_user.roles.filter(course_id=self.course.id).values_list("name", flat=True)
+        user_roles = current_user.roles.filter(course_id=self.course.id).values_list("name", flat=True)
         if action == 'allow':
             self.assertIn(rolename, user_roles)
         elif action == 'revoke':

lms/djangoapps/instructor/views/api.py

@@ -1495,10 +1495,6 @@ def update_forum_role_membership(request, course_id):
         ))
 
     user = get_student_from_identifier(unique_student_identifier)
-    target_is_instructor = has_access(user, 'instructor', course)
-    # cannot revoke instructor
-    if target_is_instructor and action == 'revoke' and rolename == FORUM_ROLE_ADMINISTRATOR:
-        return HttpResponseBadRequest("Cannot revoke instructor forum admin privileges.")
 
     try:
         update_forum_role(course_id, user, rolename, action)