STARTERKIT_theme() no longer needs to call zen_theme()

In Drupal 6, we were working around bugs/limitations of Drupal core so we had all sub-themes call zen_theme() from inside their hook_theme() implementations. This allowed Zen to do some work at theme registry build time.

In Drupal 7, we no longer need to do any of this hard-work. Its always been an awkward piece of code. Let's simplify and remove it. Since STARTERKIT_theme() would then be empty, let's remove it too.