Low: tools: Drop cluster-clean --kill-only option

This tells cluster-clean only to stop/kill all cluster daemons and,
strangely, valgrind, on all nodes in the group.

Other high-level tools like pcs, crmsh, Ansible, pssh, etc., can do
this. Pacemaker doesn't have to be able to do everything.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>