sound: Remove dead code from pcm/
No functional change intended.
Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47733
(cherry picked from commit 00731aaaed76785af8befe13e9a4e85b3554b3f5)