Don't error if deleting a non-existent pusher. (#9121)

changelog.d/9121.bugfix

@@ -0,0 +1 @@
+Fix spurious errors in logs when deleting a non-existant pusher.

synapse/storage/databases/main/pusher.py

@@ -344,7 +344,9 @@ class PusherStore(PusherWorkerStore):
                 txn, self.get_if_user_has_pusher, (user_id,)
             )
 
-            self.db_pool.simple_delete_one_txn(
+            # It is expected that there is exactly one pusher to delete, but
+            # if it isn't there (or there are multiple) delete them all.
+            self.db_pool.simple_delete_txn(
                 txn,
                 "pushers",
                 {"app_id": app_id, "pushkey": pushkey, "user_name": user_id},