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
Well I have mixed feelings about this file, from one hand it should not be located inside navigator_moz_mobile_message as it doesn't really related and having fake data in separate file sounds reasonable for me. From the other hand it duplicates asyncStorage a bit, but maybe it's fine for desktop-only file.