diskless(8): remove references to deleted clone_root script

The clone_root script was removed from the tree in commit
7736786b08e8 but the diskless(8) man page still referenced it
in two places. Remove both references.

PR: 292231
Signed-off-by: Kit Dallege <xaum.io@gmail.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/2101
(cherry picked from commit f8c8875add6946b09ea8cf1f7bbdbd90fe5b1f17)