Remove stream token-per-persister removal hack

This was an old issue that is now believed to be fixed in synapse.

synapse/handlers/sync.py

@@ -1438,16 +1438,6 @@ async def generate_sync_result(
         # Always use the `now_token` in `SyncResultBuilder`
         now_token = self.event_sources.get_current_token()
 
-        # Beeper change: remove any per-instance stream token positions, this
-        # happens when one or more event persisters fall behind the others, in
-        # practice this is very rare and usually only ~1-2 events. By removing
-        # this we dramatically simplify debugging issues, checking caches and
-        # generating room previews.
-        now_token = now_token.copy_and_replace(
-            StreamKeyType.ROOM,
-            RoomStreamToken(None, now_token.room_key.stream),
-        )
-
         if debug_current_token:
             logger.info(
                 "Overriding sync current token for debugging to: %r",