ALSA: Remove deprecated AU1X00 AC97 driver

The AU1X00 AC97 ALSA driver was deprecated in commit 7137c6bc ("ALSA:
deprecate MIPS AU1X00 AC97 driver") in favor of the newer and better ASoC
driver for the same hardware. This was almost 5 years ago and this driver
has not been in use in the mainline kernel since, it should be safe to
remove it at this point.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>