Removing Hardcore width and Height in WordPress

Add the following code in you functions.php file

// Removing Hardcore width and Height in WordPress
add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10, 3 );
function remove_thumbnail_dimensions( $html, $post_id, $post_image_id ) {
    $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
    return $html;
}