old cron jobs are not removed from crontab

[leo-schick]

When a cron job definition was removed from the MARA_CRON_JOBS list it is not removed from the crontab on next write, because we currently just create or update new cron entries but do not remove old once.

This could be fixed by iterating over all crontab entries removing which are not listed in MARA_CRON_JOBS