Update backends adjustments controller to fix order.update! call.
Fixes receiving RMA's when inventory tracking is turned off.
Fix broken spec.
Typo fix

Fixes #4814
Dont destroy all order payments in api/checkouts

Can't see why we would do that, no spec coverage or git history to
explain that behaviour either
Improving performance of migration add_line_item_id_to_spree_inventory_units

I noticed when I ran these commands on the console that the line

    shipments = Spree::Shipment.includes(:inventory_units, :order)

would load a tremendous amount of stuff from the database, negating the performance savings by the `find_each` method. This may even be a Rails bug. Putting the `includes` and the `find_each` into the same line fixed performance issues immediately.