snd_uaudio: Remove unused sc_sndstat sbuf

This information is also printed to dmesg(8) on attach.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45771

(cherry picked from commit 3402d474ceb8541d07689bad6960f90739129997)