sound: Remove dead code in pcm/ac97.c
ac97_uninitmixer() does not exist also.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D56045
(cherry picked from commit d048cec5fb41cf9529dd0b637a51c7ee72b59b3b)