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
after some thoughts, I'm fine with having a simple mock like this here. But we need to load it only if navigator.iccManager does not already exist, so maybe we can add a guard at the start of the file?