libac: drop the old implementation

farewall to the old libac. It was a fine piece of code and served
its purpose quite brilliantly, but lots of things changed meantime,
like transcode (and libac itself) being unmaintained.

A new libac will raise, reimplemented on top of ORC
http://code.entropywave.com/documentation/orc/
and will be a source-compatible drop-in replacement to the old one.