Hello, Wolfgang!
If you want to remove the featured image from the Single blog post (please correct me if I’m wrong) , there is an option in the customizer, located in Customizing->Blog->Single Post menu to turn it off. https://prnt.sc/t89c55
If for some reason this option didn’t work, insert the following custom CSS code:
.entry-header img.wp-post-image{
display: none!important;
}
Regards