http: remove unneeded cb check from setTimeout()

- This check is already covered in EventEmitter#addListener()

Refs: https://github.com/nodejs/node/pull/3618
PR-URL: https://github.com/nodejs/node/pull/3631
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/_http_outgoing.js

@@ -91,8 +91,6 @@ exports.OutgoingMessage = OutgoingMessage;
 OutgoingMessage.prototype.setTimeout = function(msecs, callback) {
 
   if (callback) {
-    if (typeof callback !== 'function')
-      throw new TypeError('callback must be a function');
     this.on('timeout', callback);
   }