sound: Do not fail from vchan_destroy() if children list is empty

vchan_destroy() should be able to call chn_kill() regardless of whether
the channel has been added to the children list yet or not. Also remove
the CHN_F_BUSY check. There is no reason we shouldn't execute the code
below if the parent is not busy.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D48185