Add a migration step to cleanup potential leftovers of bug 11833 (#12784)

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

changelog.d/12784.bugfix

@@ -0,0 +1 @@
+Delete events from the `federation_inbound_events_staging` table when a room is purged through the admin API.

synapse/storage/schema/main/delta/70/01clean_table_purged_rooms.sql

@@ -0,0 +1,19 @@
+/* Copyright 2022 The Matrix.org Foundation C.I.C
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- Clean up left over rows from bug #11833, which was fixed in #12770.
+DELETE FROM federation_inbound_events_staging WHERE room_id not in (
+    SELECT room_id FROM rooms
+);