Remove unused router method (#16149)

This PR removes a legacy router method that was used for old-style HMR, now replaced by Fast Refresh.

This method was not public:
```tsx
export type NextRouter = BaseRouter &
  Pick<
    Router,
    | 'push'
    | 'replace'
    | 'reload'
    | 'back'
    | 'prefetch'
    | 'beforePopState'
    | 'events'
    | 'isFallback'
  >
```

Even if someone found this method, it's highly unlikely they could use it successfully—it required the full module object.

packages/next/next-server/lib/router/router.ts

@@ -397,31 +397,6 @@ export default class Router implements BaseRouter {
     this.change('replaceState', url, as, options)
   }
 
-  update(route: string, mod: any) {
-    const Component: ComponentType = mod.default || mod
-    const data = this.components[route]
-    if (!data) {
-      throw new Error(`Cannot update unavailable route: ${route}`)
-    }
-
-    const newData = Object.assign({}, data, {
-      Component,
-      __N_SSG: mod.__N_SSG,
-      __N_SSP: mod.__N_SSP,
-    })
-    this.components[route] = newData
-
-    // pages/_app.js updated
-    if (route === '/_app') {
-      this.notify(this.components[this.route])
-      return
-    }
-
-    if (route === this.route) {
-      this.notify(newData)
-    }
-  }
-
   reload(): void {
     window.location.reload()
   }