Fix bug in discussion API comment update

This bug caused any attempt to update a comment with a non-null
parent_id to result in a 500 error without applying the update. The bug
was introduced in commit 2c7590d197.

lms/djangoapps/discussion_api/api.py

@@ -85,11 +85,7 @@ def _get_comment_and_context(request, comment_id):
     """
     try:
         cc_comment = Comment(id=comment_id).retrieve()
-        _, context = _get_thread_and_context(
-            request,
-            cc_comment["thread_id"],
-            cc_comment["parent_id"]
-        )
+        _, context = _get_thread_and_context(request, cc_comment["thread_id"])
         return cc_comment, context
     except CommentClientRequestError:
         raise Http404