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
so that it's easier to discuss, I've move all this in https://bugzilla.mozilla.org/show_bug.cgi?id=1069338