ACM is an ancient legacy API, and there's no point in
keeping it around for a licensed mp3 decoder now that
mp3 patents have expired and we have a decoder in -good.

We didn't ship this in cerbero anyway. If there's a good
case for the AAC encoder (which is LC only anyway) someone
should write a new plugin based on current APIs, that can
actually be built out of the box.

Fixes #850