Apply a big hammer for stale pre-OpenZFS files

-DNO_CLEAN builds have had trouble across the OpenZFS import. It's not
worth the effort to try to address this with any granularity; instead,
just trigger on a .depend file indicating a tree from before the import,
and remove the whole cddl object tree.

Reviewed by: mmacy, kevans
Differential Revision: https://reviews.freebsd.org/D26189