koji-sidetag-cleanup: delete inactive tags
Sidetags which don't have any activity (un/tagging builds) are not worth
of existence. They just trigger unnecessary repo regenerations. It
should be safe to delete them e.g. after one month. It is an additional
check compared to --old-delay and --empty-delay. These can hit the
middle ground between these two limits (empty shortest, inactive middle,
old longest).
Related: https://pagure.io/koji/issue/3293