sound: Retire pcm_veto_load

pcm_veto_load is used to prevent pcm_register() from running if the root
feeder has not been registered yet. However, feeder_register_root() is a
SYSINIT.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D47280