WP Remove default image sizes

## sgr-remove-default-image-sizes.php
<?php
/**
 * Remove standard image sizes so that these sizes are not
 * created during the Media Upload process.
 * Note: these sizes will appear "disabled" in the Media Uploader
 *
 * Hooked to intermediate_image_sizes_advanced filter
 * See wp_generate_attachment_metadata( $attachment_id, $file ) in wp-admin/includes/image.php
 *
 * @param $sizes, array of default and added image sizes
 * @return $sizes, modified array of image sizes
 * @author Ade Walker http://www.studiograsshopper.ch
 */
function sgr_filter_image_sizes( $sizes) {

	unset( $sizes['thumbnail']);
	unset( $sizes['medium']);
	unset( $sizes['large']);

	return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'sgr_filter_image_sizes');