xfce4-mixer: Remove due to GStreamer 1.x incompatibilties

After the GStreamer update, the Xfce4 mixer doesn’t start anymore.

```
$ xfce4-mixer
xfce4-mixer: error while loading shared libraries: libgstreamer-0.10.so.0: cannot open shared object file: No such file or directory
```

Rebuilding against GStreamer 1.12.x fails. The mixer is not maintained
anymore, and superseded by Xfce4-pulseaudio-plugin. Therefore, remove
it.

Fixes: commit 4127087 (Merge pull request #728 from mariux64/update-gstreamer)