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
mmm didn't get it - looks like having MockMessageManager for ThreadUI is more than enough? And IMO ThreadUI should deal with "backend" only through MessageManager.