doc: "util" is not needed to extend ES6 classes

PR-URL: https://github.com/nodejs/node/pull/9737
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/util.md

@@ -180,7 +180,6 @@ stream.write('It works!'); // Received data: "It works!"
 ES6 example using `class` and `extends`
 
 ```js
-const util = require('util');
 const EventEmitter = require('events');
 
 class MyStream extends EventEmitter {