functions.php

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