remove redundant future flag types from playground

Author: pcattori

playground/middleware/react-router.config.ts

@@ -1,11 +1,4 @@
 import type { Config } from "@react-router/dev/config";
-import type { Future } from "react-router";
-
-declare module "react-router" {
-  interface Future {
-    unstable_middleware: true;
-  }
-}
 
 export default {
   future: {