Disable ipod plugin

See https://bugs.debian.org/994758 and upstream discussion at https://github.com/doug-gilbert/sg3_utils/issues/19 where upstream basically says there is no real ABI guarantee. As the debian maintainer is nowhere to be found, it doesn't seem like libgpod can be a shared library (unless every new revision brings a new SONAME which in unpleasant).

rhythmbox(-plugins) has a transitive dependency on libsgutils2-2 via libgpod.

libgpod itself is orphaned.

And last but possibly not least, I'm not sure who still has an iPod.

Given all this I think it's better to cut ties with the stack of packages that are less then perfectly maintained.