Comment: define update_search_index() as a no-op.

Comments are not indexed, but they were being put into the "search_changes"
queue and then skipped when the message was processed.

r2/r2/models/link.py

@@ -1373,6 +1373,10 @@ class Comment(Thing, Printable):
         # Run this last
         Printable.add_props(user, wrapped)
 
+    def update_search_index(self, boost_only=False):
+        # no-op because Comments are not indexed
+        return
+
 
 class CommentScoresByLink(tdb_cassandra.View):
     _use_db = True