Add SQL delta for deleting stale pushers

changelog.d/9479.bugfix

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+Fix deleting pushers when using sharded pushers.

synapse/storage/databases/main/schema/delta/59/08delete_stale_pushers.sql

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+/* 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.
+ */
+
+
+-- Delete all pushers associated with deleted devices. This is to clear up after
+-- a bug where they weren't correctly deleted when using workers.
+DELETE FROM pushers WHERE access_token NOT IN (SELECT id FROM access_tokens);