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
Will see what I can do here, the easiest is probably putting mock_async_storage script link right after real asyncStorage one to override... even that it was nice to have all desktop-only links in one place :) Or another thought - place mock_async_storage link together with other desktop-only links, but remove real async storage link from index.html in build script