Revert "hier.7: add /lib/casper directory"

This reverts commit 407345752d0330766c86b5600740152c34be3ff0.
No longer needed since helpers are moved to /lib.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D43758