cluster: remove use of bind() in destroy()

This commit replaces process.exit.bind() with an arrow function
in Worker.prototype.destroy().

PR-URL: https://github.com/nodejs/node/pull/6502
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/cluster.js

@@ -684,10 +684,12 @@ function workerInit() {
 
   Worker.prototype.destroy = function() {
     this.exitedAfterDisconnect = true;
-    if (!this.isConnected()) process.exit(0);
+    if (!this.isConnected()) {
-    var exit = process.exit.bind(null, 0);
+      process.exit(0);
+    } else {
       send({ act: 'exitedAfterDisconnect' }, () => process.disconnect());
-    process.once('disconnect', exit);
+      process.once('disconnect', () => process.exit(0));
+    }
   };
 
   function send(message, cb) {