sound: Retire unused sndbuf_dump()
Fixes: e4e61333ffa4e90360de2dd1e4e0146f7cbf0afb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D53522
(cherry picked from commit 6d6eb25d0b19443089749e288f091bf26fc58f70)