Remove outdated constraint on remote_media_cache_thumbnails (#9725)

The `remote_media_cache_thumbnails_media_origin_media_id_thumbna_key`
constraint is superceded by
`remote_media_repository_thumbn_media_origin_id_width_height_met` (which adds
`thumbnail_method` to the unique key).

PR #7124 made an attempt to remove the old constraint, but got the name wrong,
so it didn't work. Here we update the bg update and rerun it.

Fixes #8649.

synapse/storage/databases/main/schema/delta/59/11drop_thumbnail_constraint.sql.postgres

@@ -0,0 +1,22 @@
+/* Copyright 2021 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.
+ */
+
+-- drop old constraints on remote_media_cache_thumbnails
+--
+-- This was originally part of 57.07, but it was done wrong, per
+-- https://github.com/matrix-org/synapse/issues/8649, so we do it again.
+INSERT INTO background_updates (ordering, update_name, progress_json, depends_on) VALUES
+  (5911, 'media_repository_drop_index_wo_method_2', '{}', 'remote_media_repository_thumbnails_method_idx');
+