Remove lost comment

synapse/storage/schema/delta/56/room_membership_idx.sql

@@ -13,13 +13,6 @@
  * limitations under the License.
  */
 
--- We add membership to current state so that we don't need to join against
--- room_memberships, which can be surprisingly costly (we do such queries
--- very frequently).
--- This will be null for non-membership events and the content.membership key
--- for membership events. (Will also be null for membership events until the
--- background update job has finished).
-
 -- Adds an index on room_memberships for fetching all forgotten rooms for a user
 INSERT INTO background_updates (update_name, progress_json) VALUES
   ('room_membership_forgotten_idx', '{}');