add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );
add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 );
 
function remove_thumbnail_dimensions( $html ) {
  $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
 
  return $html;
}