Remove util require

Author: jseminck

lib/rules/prop-types.js

@@ -8,7 +8,6 @@
 // https://github.com/yannickcr/eslint-plugin-react/issues/7
 
 const has = require('has');
-const util = require('util');
 const Components = require('../util/Components');
 const variable = require('../util/variable');
 const annotations = require('../util/annotations');