Remove Negative Inventory Block for Kit Products

All child products setup within a kit are currently unavailable to order if any of the products in the kit are below 0. This makes sense for static kits where you cannot send the kit in full, but for variable kits (such as apparel products) where the user can select 0 for the products without inventory, it does not make sense to stop the remaining child products from being ordered.

Note that this seems to work as suggested if inventory is exactly 0, but not if the inventory was temporarily on backorder and holds negative inventory.