Scheduler: remove empty agenda on cancel (#12989)

* Scheduler: remove empty agenda on cancel

* use iter any

* fix benches

* remove trailing None

* Add CleanupAgendas migration

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* fix ci

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Count non-empty agendas in migration

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>