Remove server from WebsocketRouter (#17238)

App contains server.
Server contains WebsocketRouter.

There is no need for WebsocketRouter to contain
server too. As is evident because the code never used it.

```release-note
NONE
```

app/websocket_router.go

@@ -16,7 +16,6 @@ type webSocketHandler interface {
 }
 
 type WebSocketRouter struct {
-	server   *Server
 	app      *App
 	handlers map[string]webSocketHandler
 }