sound: Remove unused include/linux/ac97_codec.h header

This file has been superseded by include/sound/ac97_codec.h,
and has currently no users.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>