sound: Merge pcm_chn_destroy() and chn_kill()

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