I built a simple Ionic app with the cordova splashscreen plugin before learning about iOS storyboards. Regardless, my project builds and deploys to my device fine using Xcode 9.1. However, it is showing both a splashscreen and a launch screen/storyboard. Removing the splashscreen plugin and all references to it in the project does not help, it still shows both.
Double check you removed the references in the config.xml. You might also want to remove the ios platform and add it back in, just to make sure there is nothing ‘saved’ incorrectly.