Fix returned count of delete extremities admin API (#12496)

changelog.d/12496.bugfix

+Fix bug where the admin API for [deleting forward extremities](https://github.com/matrix-org/synapse/blob/erikj/fix_delete_event_response_count/docs/admin_api/rooms.md#deleting-forward-extremities) would always return a count of 1 no matter how many extremities were deleted. Broke in v1.27.0.

synapse/storage/databases/main/events_forward_extremities.py

@@ -66,13 +66,15 @@ class EventForwardExtremitiesStore(
             """
 
             txn.execute(sql, (event_id, room_id))
+
+            deleted_count = txn.rowcount
             logger.info(
                 "Deleted %s extra forward extremities for room %s",
-                txn.rowcount,
+                deleted_count,
                 room_id,
             )
 
-            if txn.rowcount > 0:
+            if deleted_count > 0:
                 # Invalidate the cache
                 self._invalidate_cache_and_stream(
                     txn,
@@ -80,7 +82,7 @@ class EventForwardExtremitiesStore(
                     (room_id,),
                 )
 
-            return txn.rowcount
+            return deleted_count
 
         return await self.db_pool.runInteraction(
             "delete_forward_extremities_for_room",