Remove an assert on _need_refresh()

The chance of us needing a refresh at this point is TINY, but it's probably
better to return a list of relays stale by a millisecond than it is to cause
the user's program to crash.

sbws/lib/relaylist.py

@@ -153,7 +153,6 @@ class RelayList:
                     log.debug('No we don\'t need to refresh our relays. '
                               'It was done by someone else.')
             log.debug('Giving back the lock for refreshing relays.')
-        assert not self._need_refresh()
         return self._relays
 
     @property