util: Remove exec, has been deprecated for years

PR-URL: https://github.com/nodejs/node/pull/2530
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/util.js

@@ -838,11 +838,6 @@ function hasOwnProperty(obj, prop) {
 
 // Deprecated old stuff.
 
-exports.exec = internalUtil.deprecate(function() {
-  return require('child_process').exec.apply(this, arguments);
-}, 'util.exec is deprecated. Use child_process.exec instead.');
-
-
 exports.print = internalUtil.deprecate(function() {
   for (var i = 0, len = arguments.length; i < len; ++i) {
     process.stdout.write(String(arguments[i]));