sys-process/cronie: fix USE=anacron mess
run-crons script from sys-process/cronbase is duplicating anacron's
functionality. And since we run run-crons every 10 minutes and anacron
every hour, we end up running every daily/weekly/monthly cron job twice.
Fix this by removing run-crons from crontab.
Also enable USE=anacron by default because this is closer to behaviour we
previously had with USE=-anacron (thanks to run-crons).
Closes: https://bugs.gentoo.org/621706
Closes: https://github.com/gentoo/gentoo/pull/7079