ALSA: au88x0 - Remove unused "Master Mono" Playback Volume and Playback Switch of ac97 codec

Remove "Master Mono Playback Volume" and "Master Mono Playback Switch"
of ac97 mixer since au88x0 does no use "Master Mono Pin" of AC97 codec
even au88x0 support mono playback

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>