ALSA: hda - Disable AA-mix on Sony Vaio S13

The analog-loopback causes the speaker noises even if it's set to zero
volume.  As a simple workaround, just get rid of the loopback mixer.

Bugzilla: https://bugzilla.novell.com/show_bug.cgi?id=873704


Signed-off-by: Takashi Iwai <tiwai@suse.de>