depend-cleanup: Add a clean_obj function

The clean_dep function is primarily intended for when a source file is
renamed. The 20241025 entry for libcrypt was not related to a rename
and resulted in libcrypt getting cleaned out and rebuilt every time.
Add a clean_obj function which does what the 20241025 entry needs:
delete an object file if it matches a particular regex.

This also changes every occurrence of rm -f to rm -fv so we can
more easily tell what gets cleaned up.

Fixes: d8cd2d0833e0 ("depend-cleanup.sh: clean up after hash function removal from libcrypt")
Reviewed by: jhb, emaste
Differential Revision: https://reviews.freebsd.org/D52012