Clean up unused tables for unadopted racks/sleds

New tables were added in #11548 that are not used, and since we've decided to switch to an inventory-based system for tracking unadopted racks and sleds, the planned uses for these tables will not be implemented. When I get a chance, I'll make a new PR to drop them, but I'm trying to prioritize other things right now so this issue just serves as a reminder to get back to this later.