ccdconfig: remove obsolete references to BSD disklabels

ccd(4) previoulsy had knowledge of BSD disklabels, and relied on their
use on the underlying disks, but this hasn't been the case since 2003
(commit 0f76d6d822f4).

Remove disklabel references from the man page.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43574