Remove Featured Image as Header Image in Posts

Hello,

First, thank you for your theme. It’s great!
I’d like to remove the Featured Image showing up as the Header Image of Posts. I’ll be using the Featured Image in the content of the post, and for the Home Page but I would like the Header to be the same as the home page and across all Posts. Let me know if you need more detail.
For information, I’ve created a child theme.

Could you help me?
Thanks

Maudoune