reprepro-related cron mails shouldn't be sent to sysadmins
Originally created by @groente on #14738 (Redmine)
reprepro-time-based-snapshots cronjobs currently mail to
tails-sysadmins, whereas tails-rm would be a more suitable destination.
Same for the “Deleting vanished identifier” email from
tails-update-reprepro-config.