Remove featured image width/ height

Add this to the theme functions file to remove width and height attributes from featured images.

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