Retire old diskless setup scripts

These scripts predate /etc/rc.diskless* and use a different scheme. A
comment was added to them back in 2002 noting they were 3 years old at
that point.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D41951

(cherry picked from commit 7736786b08e8c9edb07088db3229695d0afed600)
(cherry picked from commit f97a3f6a017103b2afaf451af055ec4131a455a9)