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
Probably we can, but it's in shared sim picker component (that's is going to be replaced with webcomponent soon btw).

Can you point me to the code where you add mocks for DSDS? Just to understand better what suits for your simulator. 

To be honest I'm eager to make this "mocking" as transparent as possible so that real code doesn't have to be patched and deal with various ```if``` branches... Maybe we can workout better solution