ALSA: ac97: remove unused variable 'snd_ac97_controls_master_mono'

sound/pci/ac97/ac97_codec.c:599:38: warning:
 snd_ac97_controls_master_mono defined but not used [-Wunused-const-variable=]

It is never used, so can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>