docs: Improve routing validate example (#18728)

docs/1.getting-started/5.routing.md

@@ -132,7 +132,6 @@ If you have a more complex use case, then you can use anonymous route middleware
 <script setup>
 definePageMeta({
   validate: async (route) => {
-    const nuxtApp = useNuxtApp()
     // Check if the id is made up of digits
     return /^\d+$/.test(route.params.id)
   }