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
it's kind of silly regexp here :) Didn't think too much for the desktop-only code, but we'll be happy to replace with something more smart :)