Control-click on an unwanted iPad launch image in the Summary window and choose Delete. This will remove the reference to the image as a launch image for iPad. Even better, go ahead and include solid black PNG images for the unneeded launch images. They will be compressed by Xcode when it builds your app, so that they will take up virtually no space in the app bundle.
I regard the fact that Xcode automatically creates default launch images for iPhone but not iPad as a bug. I suggest you complain about this behavior at bugreporter.apple.com.
EDIT: Just to be clear, what you want is a minimal set of properly sized images with the following names:
This will prevent Xcode from using e.g. an iPhone launch image in an iPad slot.