Remove postprocessors from unprocessed asset

Unprocessed assets are evaluated using all postprocessors but without any engines. It causes unexpected behavior and errors because postprocessor expects data allready parsed  by an engine.

I think simply removing postprocessors and leaving just preprocessors and engines for unprocessed assets shouldn't cause any problems.

I found this issue while using https://github.com/ai/autoprefixer-rails. Great gem and i really hope it gets the support it deserves, including compatibility with this also very useful gem.