sj-hierarchy.pl: list artifacts w/o maven-helper

The "maven-helper.sh artifacts" command is currently broken.
Let's just do it ourselves, to avoid future breakages.
We eventually want to get rid of maven-helper.sh, anyway.
maven-helper.sh: remove broken artifacts command

After commit b54b1e36676516f6995044ee2937334c355fa158, the "artifacts"
command stopped working. Since it was only used by sj-hierarchy.pl, and
that script now does the work without assistance from maven-helper, we
can just remove the artifacts command to simplify our lives.