sound: Merge pcm_chn_destroy() and chn_kill()

[christos]

Summary

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

[markj]

sys/dev/sound/pcm/channel.c

1285–1288

There is something wonky going on with the indentation here.