I don’t support your idea of avoiding launch images as they are there for a reason, but if you really wanna do it for some reason, you should use the [email protected]
file (640×1136 px) like the one you get when creating a new project. It’s just solid black, doesn’t take up a lot of disk space and will look as if there are no launch images whatsoever.
Besides setting a 4-inch retina launch image, there is currently no known way to get full 1136 px height on the new screen.