ALSA: HDA: remove useless mixers on Aspire 8930G

This patch removes some extra mixers that do nothing on the Acer Aspire
8930G.

The CD mixer is useless because the SATA DVD/Blu-Ray drive has no analog
audio output, and the Side mixer is useless because we max out at 6ch
anyway.

Signed-off-by: Hector Martin <hector@marcansoft.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>