Remove Default Wordpress Thumbnail Sizes

functions.php

//Remove default thumbanail sizes on all blogs (works on Wordpress multisite also)
<?php
function remove_default_image_sizes( $sizes) {
    unset($sizes['thumbnail']);
    unset($sizes['medium']);
    unset($sizes['large']);
    return $sizes;
}
add_filter('intermediate_image_sizes_advanced','remove_default_image_sizes');