remove shutdown method on the Connection (#4249)

There's no need to have a dedicated shutdown method, as the use case
(shutting down an outgoing connection attempt on context cancellation)
can be achieved by using Connection.destroy.

packet_handler_map.go

@@ -191,7 +191,6 @@ func (h *packetHandlerMap) ReplaceWithClosed(ids []protocol.ConnectionID, pers p
 
 	time.AfterFunc(h.deleteRetiredConnsAfter, func() {
 		h.mutex.Lock()
-		handler.shutdown()
 		for _, id := range ids {
 			delete(h.handlers, id)
 		}