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
mmmm yeah ```serializeNewHTMLDocumentOutput``` looks monstrous, maybe we can do as always - keep it simple for now and extend if we really need it? So in case we need to make it more complex or add one more regex - we'll make it smarter.

>  I'd prefer if the build system would pass us a document and take care of the parse/serialize. We can maybe file a bug for this?

Mmm we may have several html files that we'd like to process, probably we just need single handy module that can process it under the hood, it reminds me C# XDocument that can read from stream (eg. file) to XML Object Model and save serialized content to another stream afterwards.