Remove/Disable WordPress Featured Images/Thumbnails in functions.php or plugin

gistfile1.txt

/* 
	Disable auto-generated thumbnails.  Via 
	- https://wordpress.stackexchange.com/questions/91166/how-to-disable-wordpress-from-creating-thumbnails
	- https://wordpress.stackexchange.com/questions/260609/remove-image-size-doesnt-seem-to-be-working
*/
function add_image_insert_override($sizes){
    unset( $sizes['thumbnail']);
    unset( $sizes['medium']);
    return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'add_image_insert_override' );

function remove_image_sizes() {
if ( function_exists( 'add_image_size' ) ) { 
    remove_image_size( 'et-pb-portfolio-image');
    remove_image_size( 'et-pb-portfolio-module-image');
    remove_image_size( 'et-pb-portfolio-image-single');
    remove_image_size( 'et-pb-gallery-module-image-portrait');
    remove_image_size( 'et-pb-post-main-image-fullwidth-large');
    remove_image_size( 'et-pb-image--responsive--desktop');
    remove_image_size( 'et-pb-image--responsive--tablet');
    remove_image_size( 'et-pb-image--responsive--phone');
 }
}
add_action('after_setup_theme', 'remove_image_sizes');