Remove get_destination_retry_timings cache

Currently we rely on the master to invalidate this cache promptly.
However, after having moved most federation endpoints off of master this
no longer happens, causing outbound fedeariont to get blackholed.

Fixes #3798

synapse/storage/transactions.py

@@ -156,7 +156,6 @@ def delivered_txn(self, transaction_id, destination, code, response_dict):
         """
         pass
 
-    @cached(max_entries=10000)
     def get_destination_retry_timings(self, destination):
         """Gets the current retry timings (if any) for a given destination.
 
@@ -212,10 +211,6 @@ def _set_destination_retry_timings(self, txn, destination,
                                        retry_last_ts, retry_interval):
         self.database_engine.lock_table(txn, "destinations")
 
-        self._invalidate_cache_and_stream(
-            txn, self.get_destination_retry_timings, (destination,)
-        )
-
         # We need to be careful here as the data may have changed from under us
         # due to a worker setting the timings.