Don't insert without deduplication. In this case we never actually use this table, so simply remove the insert entirely

synapse/storage/events.py

@@ -127,28 +127,6 @@ class EventsStore(SQLBaseStore):
                 ],
             )
 
-        if event.is_state() and is_new_state:
-            if not backfilled and not context.rejected:
-                self._simple_insert_txn(
-                    txn,
-                    table="state_forward_extremities",
-                    values={
-                        "event_id": event.event_id,
-                        "room_id": event.room_id,
-                        "type": event.type,
-                        "state_key": event.state_key,
-                    },
-                )
-
-                for prev_state_id, _ in event.prev_state:
-                    self._simple_delete_txn(
-                        txn,
-                        table="state_forward_extremities",
-                        keyvalues={
-                            "event_id": prev_state_id,
-                        }
-                    )
-
         outlier = event.internal_metadata.is_outlier()
 
         if not outlier: