Update _app.tsx

Also moved container import after removing that component. As per original commit note.

Author: CodenameCookie

pages/_app.tsx

@@ -1,7 +1,7 @@
 import CssBaseline from '@material-ui/core/CssBaseline';
 import Grid from '@material-ui/core/Grid';
 import { ThemeProvider } from '@material-ui/styles';
-import App, { Container } from 'next/app';
+import App from 'next/app';
 import Head from 'next/head';
 import React from 'react';