I just removed that file, recompiled cron, purged the current one and
re-installed it. And guess what, no /lib/systemd/system/cron.service
anymore :)
I always get the impression that there should be is a copy command on
one of the compile scripts. So I think removing the file is not a good
solution, in case Devuan must support systemd :)
I am sorry for all the noise. I should be more carefull next time.