Remove unused events_by_room (#10421)

It looks like it was first used and introduced in 5130d80#diff-8a4a36a7728107b2ccaff2cb405dbab229a1100fe50653a63d1aa9ac10ae45e8R305 but the 

But the usage was removed in 4c6a31c#diff-8a4a36a7728107b2ccaff2cb405dbab229a1100fe50653a63d1aa9ac10ae45e8

Author: MadLittleMods

changelog.d/10421.misc

@@ -0,0 +1 @@
+Remove unused `events_by_room` code (tech debt).

synapse/storage/databases/main/events.py

@@ -2010,10 +2010,6 @@ def _update_backward_extremeties(self, txn, events):
 
         Forward extremities are handled when we first start persisting the events.
         """
-        events_by_room: Dict[str, List[EventBase]] = {}
-        for ev in events:
-            events_by_room.setdefault(ev.room_id, []).append(ev)
-
         query = (
             "INSERT INTO event_backward_extremities (event_id, room_id)"
             " SELECT ?, ? WHERE NOT EXISTS ("