Fix up changelog and remove spurious comment

changelog.d/3914.bugfix

-Fix bug where outboud federation would stop talking to some servers when using workers
+Fix bug where outbound federation would stop talking to some servers when using workers

synapse/storage/transactions.py

@@ -196,8 +196,6 @@ class TransactionStore(SQLBaseStore):
             retry_interval (int) - how long until next retry in ms
         """
 
-        # XXX: we could chose to not bother persisting this if our cache thinks
-        # this is a NOOP
         return self.runInteraction(
             "set_destination_retry_timings",
             self._set_destination_retry_timings,