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
Oh, I was thinking about this too, but had doubts that was done intentionally to keep data between launches. I'd prefer to have asyncStorage mock with in-memory-store. Can do this in this patch