Remove ffmpeg decoder plugin

It's only used when writing AAC, WMA, WavPack and APE to a CD-Audio and it
doesn't even seem to work nowadays (i get audio garbage for half [*], the
other half don't even let you drag them to the view) and on top of that
with ffmpeg5 doesn't even compile.

Removing for good, someone will need to rewrite it if they care about
those file formats

[*] My interpretation is that it assumes the output will be in a very
specific format but doesn't necessarily is