Scheduler pallet bloats `Agenda` too easily when there's lots of insertions/removals

[gavofyork]

Right now, deleting a previously scheduled item results in a None "hole" in its Agenda for that block, which is not used since Scheduler just appends on Agenda when adding a new item. If items are continuously inserted and deleted then this can bloat storage and become a problem.

Instead, None items in the Agenda should be inserted into or the entire mechanism replaced with remove or swap_remove.

[ggwpez]

I will tackle this in #11589 together with bound storage

[ggwpez]

I think there is still a case where through preemption or re-scheduling an agenda can be all None.
But we could add a simple perpetual migration that clears out the all-None empty agendas on every runtime upgrade. @gavofyork