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
Without these mock, I see annoying exceptions from l10n date helper :) Maybe we can utilize this mock to test settings dependent code in browser too.