aout: remove what should be an unneeded update of "mute"

if aout_MuteSet returns successfully, the "mute" variables at the aout
and playlist levels have already been updated via a call to aout_MuteReport.

This late update was a hack to make things work when a audio output driver
forgets to call aout_MuteReport. So this patch may bring regression for those
audio output drivers that still forget to call aout_MuteReport and yet
terminate successfully. This is likely to occur in corner cases, for instance
when no stream is available as was the case for pulse.c

src/playlist/aout.c

@@ -120,8 +120,6 @@ int playlist_MuteSet (playlist_t *pl, bool mute)
     {
         ret = aout_MuteSet (aout, mute);
         vlc_object_release (aout);
-        if (ret == 0)
-            var_SetBool (pl, "mute", mute);
     }
     return ret;
 }