sound: Merge pcm_chn_destroy() and chn_kill()

pcm_chn_destroy() acts like a wrapper around chn_kill(), and
additionally calls a few more functions that should in fact be part of
chn_kill()'s logic. Merge pcm_chn_destroy()'s functionality in
chn_kill() to improve readability, as well as code layering.

While here, convert chn_kill() to void as it currently always returns 0.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44984