docs: Remove Raspbmc and XBian workarounds
docs: Remove Raspbmc and XBian workarounds

Ref https://github.com/mopidy/mopidy/issues/1326#issuecomment-172323148

Raspbmc is replaced by OSMC that works with Mopidy without conflicts.

XBian I'm not sure about, but the workaround is still searchable in
older version of our docs, and we'll soon be moving to GStreamer 1.x,
probably affecting the dep on libtag.

vrs01/mopidy,jodal/mopidy,adamcik/mopidy,mokieyue/mopidy,ZenithDK/mopidy,kingosticks/mopidy,mokieyue/mopidy,jodal/mopidy,tkem/mopidy,mokieyue/mopidy,tkem/mopidy,mopidy/mopidy,tkem/mopidy,vrs01/mopidy,ZenithDK/mopidy,vrs01/mopidy,ZenithDK/mopidy,mopidy/mopidy,mopidy/mopidy,adamcik/mopidy,ZenithDK/mopidy,tkem/mopidy,kingosticks/mopidy,mokieyue/mopidy,jcass77/mopidy,kingosticks/mopidy,vrs01/mopidy,adamcik/mopidy,jodal/mopidy,jcass77/mopidy,jcass77/mopidy