In Xcode 10, the "Copy Swift standard libraries" script is done last — after any custom build phases that may be added to the target. Hence it is not possible to create a custom build script to specifically remove Swift standard dylib files added by Xcode.
The question is, how to remove some of these standard libraries that are erroneously added by Xcode?
The reason behind it is that when Xcode 10 builds a Swift 4.2 macOS app that has a Photo Editing extension, it adds libswiftMapKit.dylib into the application bundle — even when the application does not have Maps functionality or import MapKit. This caused an App Review rejection.