Bug 1050416 - [Messages][Build] Add build script to ignore the desktop-only folder for production build. r=schung

* Added build script to delete desktop-only folder;
* Removed desktop-only "if"s from code;
* Changed way of loading desktop-only shims
Ok I see... Let me put here some my thoughts about simulator that I had some time ago to start discussion :) What if:
* We have links to all shims in index.html (that are removed by build.js for device);
* All shims still responsible only for it's own matters;
* We have central ```desktop-only/simulator.js``` that has the following responsibilities:
 * Build and manage simulator UI in full. Currently I see that it's built by data from several "providers", not sure if need that level of abstraction;
 * Eg. listen for window.onKeyDown for some magic key combination to show Simulator panel as overlay. In that case we won't have iframe and can use responsive view (though value is arguable)
 * Somehow notifies mocks about relevant changes in Simulator UI (like Simulator UI (Settings) -> navigator_moz_settings -> call mozSettings observers). Shouldn't be difficult as we're in the same document.
 * Simulator UI changes can be saved via modifying query params or sessionStorage for example to persist between reloads..

I haven't analyzed code too much, maybe I'm missing the point of having it like this. It would be great to hear it from you!