Somewhere in your theme or parent theme, there’s this line:
add_theme_support( 'post-thumbnails' );
If it’s in your child theme, you can simply remove it. If it’s in your parent theme, you can add this to your child theme functions.php file.
// in your Child Theme's functions.php
// Use the after_setup_theme hook with a priority of 11 to load after the
// parent theme, which will fire on the default priority of 10
add_action( 'after_setup_theme', 'remove_featured_images_from_child_theme', 11 );
function remove_featured_images_from_child_theme() {
// This will remove support for post thumbnails on ALL Post Types
remove_theme_support( 'post-thumbnails' );
// Add this line in to re-enable support for just Posts
add_theme_support( 'post-thumbnails', array( 'post' ) );
}
https://codex.wordpress.org/Function_Reference/remove_theme_support