sound: Remove unused CHN_F_SILENCE

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D47664

(cherry picked from commit 6d77827b963ec1a72de3f46d51fe14ed30ef437e)