sound: Remove leftover references to snd_clone

The snd_clone framework does not exist as of
e8c0d15a64fadb4a330f2da7244becaac161bb70 ("sound: Get rid of snd_clone
and use DEVFS_CDEVPRIV(9)"), so remove leftover references to it from
unit.c.

Sponsored by: The FreeBSD Foundation
MFC after: 2 months
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44758