sound: Remove SNDBUF_LOCKASSERT()
It's a no-op. There is no lock associated with the buffer.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D48008
(cherry picked from commit 6524d2a5affd02623e6909fff0ca03ba21882a0c)