Don't check for zmdc.pl when stopping via systemd

If this check ever actually returns true, it makes ExecStop a no-op
and systemd will forcefully kill all processes in the service
see man systemd.service

This also prevents a deadlock when running systemctl stop,
zmpkg.pl detects bash as it's parent and attempts to call
systemctl stop itself, which blocks forever waiting for the
task it's in to complete