sound: Remove useless newspd check in sysctl_dev_pcm_vchanrate()

feeder_rate_min functions as the lower boundary.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D46834

(cherry picked from commit f3092614bcae949332a8f21f7b78ba68b3ee5899)