sound: Remove redundant sndbuf_free() in chn_kill()

If b exists, it will get destroyed by sndbuf_destroy() (which calls
sndbuf_free()) a few lines below, so the additional sndbuf_free() here
is redundant.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53527