frontend: drop delete_after tag for re-activated EOLed chroot
    
    It's better to reset the `delete_after` timestamp, otherwise our cron
    jobs could mistakenly think that the re-activated chroot is still meant
    to be garbage-collected.
    
    Also, don't use the update() wrapper and set the delete_after attribute
    directly (delete_after=None in update() has no effect).