Remove support for sort_order (order_direction).

- Default of "desc" is all that is in use, and remains as-is.
- Discussion API keeps order_direction param, to minimize changes,
but it only accepts "desc".
- Discussion webapp simply no longer sends in the sort order.

lms/djangoapps/discussion_api/views.py

@@ -155,8 +155,9 @@ class ThreadViewSet(DeveloperErrorViewMixin, ViewSet):
             "vote_count". The key to sort the threads by. The default is
             "last_activity_at".
 
-        * order_direction: Must be "asc" or "desc". The direction in which to
-            sort the threads by. The default is "desc".
+        * order_direction: Must be "desc". The direction in which to sort the
+            threads by. The default and only value is "desc". This will be
+            removed in a future major version.
 
         * following: If true, retrieve only threads the requesting user is
             following
@@ -164,6 +165,7 @@ class ThreadViewSet(DeveloperErrorViewMixin, ViewSet):
         * view: "unread" for threads the requesting user has not read, or
             "unanswered" for question threads with no marked answer. Only one
             can be selected.
+
         * requested_fields: (list) Indicates which additional fields to return
           for each thread. (supports 'profile_image')