ext/ffmpeg/: Don't register the WavPack demuxer and decoder. They don't work, we have better ones and the output of t...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_register):
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_register):
Don't register the WavPack demuxer and decoder. They don't work,
we have better ones and the output of the demuxer/input of the
decoder is in a different format than what audio/x-wavpack of the
wavpack plugin is (it seems that the demuxer strips of the wavpack
headers from every frame).
This fixes typefinding of Wavpack files again, as the ffmpeg
typefinder was preffered for some reason and gave
application/x-gst_ff-wv instead of audio/x-wavpack.