sndctl(8): Remove unnecessary val variable from sysctl functions

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50397

(cherry picked from commit ac86e65e0e25a3e3fd6f91b17d2db874cea83c3b)