remove dssi-vst from audio-plugins seed

ubuntustudio-meta is not currently building on the 64 bit architecture due to the ia32-libs transition to depend on multiarch libraries. removing the dssi-vst package from the 'audio-plugin' seed should allow ubuntustudio-meta to be built.

the dependencies chain is as follows:
ubuntustudio-meta -> dssi-vst -> wine-1.3 -> ia32-libs

colin explains in an email [1], " ia32-libs is being transitioned to depend on appropriate multiarch libraries (avoiding the giant maintenance headache of ia32-libs is a major goal of the multiarch project for 12.04); but germinate can't traverse multiarch
dependency chains, so it doesn't know that it needs to include ia32-libs-multiarch:i386 (which ia32-libs:amd64 depends on) and all the i386 libraries that ia32-libs-multiarch in turn depends on.

Even if germinate did know how to do this, and we figured out some appropriate output format, we'd still have to extend cdimage to recognise that it needed to copy the appropriate packages onto the image, and we'd have to figure out how Task fields ought to work for that in Launchpad (which doesn't have a clear answer as far as I can see, although we might be able to skate around the problem)."

[1] https://lists.ubuntu.com/archives/ubuntu-release/2011-November/000482.html