Fix the sky build instructions

sky/HACKING.md

 Hacking on Sky
 ==============
 
 Building
 --------
 
 * Follow the setup & build instructions for [Mojo](https://github.com/domokit/mojo)
-* Build ``sky`` with ``ninja``, e.g. ``ninja -C out/Debug sky`` for a debug build.
-
-If you want to build release, run the following commands:
-
-* ``gn gen out/Release --args="is_debug=false"``
-* ``ninja -C out/Release sky``
 
 Running applications
 --------------------
 
 * ``./sky/tools/skydb --debug [url]``
   * You should see a ``(skydb)`` prompt
   * Type ``help`` to see the list of available commands
   * Note: skydb is currently hard-coded to use ``//out/Debug``
 
 * ``./sky/tools/test_sky --debug``
@@ skipping 16 matching lines @@
 * Write tests in [mocha format](http://visionmedia.github.io/mocha/#getting-started) and use [chai asserts](http://chaijs.com/api/assert/):
 ```html
 describe('My pretty test of my subject', function() {
   var subject = new MySubject();
 
   it('should be pretty', function() {
     assert.ok(subject.isPretty);
   });
 });
 ```