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
> Mmm we may have several html files that we'd like to process

yeah, but we never need to process them in one call, so the build system could call us several times.