Ensure that remove_pusher is always async (#7981)

changelog.d/7981.misc

@@ -0,0 +1 @@
+Convert various parts of the codebase to async/await.

synapse/app/generic_worker.py

@@ -628,7 +628,7 @@ class GenericWorkerServer(HomeServer):
 
         self.get_tcp_replication().start_replication(self)
 
-    def remove_pusher(self, app_id, push_key, user_id):
+    async def remove_pusher(self, app_id, push_key, user_id):
         self.get_tcp_replication().send_remove_pusher(app_id, push_key, user_id)
 
     def build_replication_data_handler(self):