Thanks for contacting us with your question!
Unfortunately, the API does not allow you to easily disable these lines. If you really care about it, you can do it with a wrap workaround, as in the demo below.
If you would like to add that option to become a feature, you can request it in our official GitHub repository.
Link: https://github.com/highcharts/highchart ... new/choose
I hope you will find it useful