Yep - even more painful with the new iOS 7 sizes!
If you don't re-use the launchimage files elsewhere in your app, you could use a postbuild hook to clean up those assets before we create the app. Something like this:
import glob
import shutil
import sys
def main(platform):
if platform == "ios":
for launch_dir in glob.glob("ios/*/assets/src/launch"):
shutil.rmtree(launch_dir)
if __name__ == "__main__":
main(sys.argv[1])