Delete event_push_summary_unique_index again.

changelog.d/14669.bugfix

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+Fix a bug introduced in Synapse 1.70.0 which could cause spurious `UNIQUE constraint failed` errors in the `rotate_notifs` background job.

synapse/storage/schema/main/delta/73/23_fix_thread_index.sql

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+/* 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.
+ */
+
+-- If a Synapse install made a large jump in versions (from < 1.62.0 to >= 1.70.0)
+-- in a single upgrade then it might be possible for the event_push_summary_unique_index
+-- to be added from delta 71/02event_push_summary_unique.sql and not deleted in
+-- 73/06thread_notifications_thread_id_idx.sql.
+--
+-- See https://github.com/matrix-org/synapse/issues/14641
+
+-- Stop this from happening on newly updated Synapses.
+DELETE FROM background_updates WHERE update_name = 'event_push_summary_unique_index';
+
+-- Fix broken Synapses.
+DROP INDEX IF EXISTS event_push_summary_unique_index;