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
yurenju was talking about using ```if (options.PRODUCTION === '1')``` here, but I think we don't need it as it works in browser and always removed from package when pushed to device. Or am I missing something?