sound: Retire snd_dbuf->dev

Redundant, and if we at some point really need this, we can fetch it
from snd_dbuf->channel->dev.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53519

(cherry picked from commit 55540ebc33ad9a52bc4de121cb598243153981a8)