unnecessary prop store in getInitialProps (#5983)

examples/with-mobx-state-tree/pages/_app.js

@@ -5,7 +5,7 @@ import App, { Container } from 'next/app'
 import { initializeStore } from '../stores/store'
 
 export default class MyApp extends App {
-  static async getInitialProps ({ Component, router, ctx }) {
+  static async getInitialProps ({ Component, ctx }) {
     //
     // Use getInitialProps as a step in the lifecycle when
     // we can initialize our store