Overwrite default image sizes.

functions.php

<?php
// Overwrite default image sizes.
add_action('init', 'smntcs_remove_image_sizes');
function smntcs_remove_image_sizes() {
	add_image_size( 'course_archive_thumbnail', 0 );
	add_image_size( 'course_single_thumbnail', 0 );
	add_image_size( 'lesson_archive_thumbnail', 0 );
	add_image_size( 'lesson_single_thumbnail', 0 );
	add_image_size( 'portfolio_horizontal', 0 );
	add_image_size( 'portfolio_slider', 0 );
	add_image_size( 'portfolio_vertical', 0 );
	add_image_size( 'shop_catalog', 0 );
	add_image_size( 'shop_thumbnail', 0 );
	add_image_size( 'woocommerce_gallery_thumbnail', 0 );
	add_image_size( 'woocommerce_single', 0 );
	add_image_size( 'woocommerce_thumbnail', 0 );
}