Remove `Generator` in `_purge_unreferenced_state_groups` twice (#17815)

Context: https://github.com/matrix-org/synapse/issues/15439
(https://github.com/element-hq/synapse/issues/15439)

Also see discussion in https://github.com/element-hq/synapse/pull/17813

changelog.d/17815.bugfix

+Avoid lost data on some database query retries.

synapse/storage/databases/state/store.py

@@ -804,11 +804,11 @@ class StateGroupDataStore(StateBackgroundUpdateStore, SQLBaseStore):
         logger.info("[purge] removing redundant state groups")
         txn.execute_batch(
             "DELETE FROM state_groups_state WHERE state_group = ?",
-            ((sg,) for sg in state_groups_to_delete),
+            [(sg,) for sg in state_groups_to_delete],
         )
         txn.execute_batch(
             "DELETE FROM state_groups WHERE id = ?",
-            ((sg,) for sg in state_groups_to_delete),
+            [(sg,) for sg in state_groups_to_delete],
         )
 
     @trace