Trim on_civzone_delete logic

[myk002]

In the Buildings module, on_civzone_delete cleans civzone relationships before delete. DF now handles most of that itself, and we can remove some logic.

From Quietust on Discord:
Though I don't know how much of on_civzone_delete is actually necessary, since building_civzonest::removeUses cleans itself from all building.relations[] and also unassigns all units/items.
delete_civzone_squad_links and delete_assigned_unit_links are probably still necessary.