don't close websocket when mailbox is deleted

This made sense for ServerSentEvent channels (which has no purpose once
the channel was gone), but not so much for websockets. And it prevented
testing duplicate-close.

src/wormhole/server/rendezvous_websocket.py

@@ -194,7 +194,7 @@ class WebSocketRendezvous(websocket.WebSocketServerProtocol):
         def _send(event):
             self.send("message", message=event)
         def _stop():
-            self._reactor.callLater(0, self.transport.loseConnection)
+            pass
         for old_message in self._mailbox.add_listener(self, _send, _stop):
             _send(old_message)