Remove acmmp3dec and acmenc plugins

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
7 jobs for meson-build-more-win-plugins in 31 minutes and 23 seconds (queued for 12 minutes and 51 seconds)
Status Job ID Name Coverage
  Preparation
passed #66524
gst indent

00:00:32

passed #66523
manifest

00:00:23

 
  Build
passed #66525
build fedora x86_64

00:03:38

 
  Test
passed #66575
check fedora

00:16:37

failed #66526
check fedora

00:17:51

 
  Full Builds
passed #66527
build android arm64 api28

00:05:46

passed #66528
build cerbero fedora x86_64

00:10:12