Remove image classes from post thumbnail output

You might try something like this in your functions.php:

//remove class from the_post_thumbnail
function the_post_thumbnail_remove_class($output) {
        $output = preg_replace('/class=".*?"https://wordpress.stackexchange.com/", '', $output);
        return $output;
}
add_filter('post_thumbnail_html', 'the_post_thumbnail_remove_class');