Remove was_forgotten_at

This is unused. IT MUST DIE!!!1
̧̪͈̱̹̳͖͙H̵̰̤̰͕̖e̛ ͚͉̗̼̞w̶̩̥͉̮h̩̺̪̩͘ͅọ͎͉̟ ̜̩͔̦̘ͅW̪̫̩̣̲͔̳a͏͔̳͖i͖͜t͓̤̠͓͙s̘̰̩̥̙̝ͅ ̲̠̬̥Be̡̙̫̦h̰̩i̛̫͙͔̭̤̗̲n̳͞d̸ ͎̻͘T̛͇̝̲̹̠̗ͅh̫̦̝ͅe̩̫͟ ͓͖̼W͕̳͎͚̙̥ą̙l̘͚̺͔͞ͅl̳͍̙̤̤̮̳.̢
̟̺̜̙͉Z̤̲̙̙͎̥̝A͎̣͔̙͘L̥̻̗̳̻̳̳͢G͉̖̯͓̞̩̦O̹̹̺!̙͈͎̞̬ *

synapse/storage/roommember.py

@@ -578,7 +578,6 @@ class RoomMemberStore(RoomMemberWorkerStore):
             )
             txn.execute(sql, (user_id, room_id))
 
-            txn.call_after(self.was_forgotten_at.invalidate_all)
             txn.call_after(self.did_forget.invalidate, (user_id, room_id))
             self._invalidate_cache_and_stream(
                 txn, self.who_forgot_in_room, (room_id,)
@@ -609,31 +608,6 @@ class RoomMemberStore(RoomMemberWorkerStore):
         count = yield self.runInteraction("did_forget_membership", f)
         defer.returnValue(count == 0)
 
-    @cachedInlineCallbacks(num_args=3)
-    def was_forgotten_at(self, user_id, room_id, event_id):
-        """Returns whether user_id has elected to discard history for room_id at
-        event_id.
-
-        event_id must be a membership event."""
-        def f(txn):
-            sql = (
-                "SELECT"
-                "  forgotten"
-                " FROM"
-                "  room_memberships"
-                " WHERE"
-                "  user_id = ?"
-                " AND"
-                "  room_id = ?"
-                " AND"
-                "  event_id = ?"
-            )
-            txn.execute(sql, (user_id, room_id, event_id))
-            rows = txn.fetchall()
-            return rows[0][0]
-        forgot = yield self.runInteraction("did_forget_membership_at", f)
-        defer.returnValue(forgot == 1)
-
     @defer.inlineCallbacks
     def _background_add_membership_profile(self, progress, batch_size):
         target_min_stream_id = progress.get(