This field is really unnecessary, no major online store I know of uses anything like it. So away with it.
if (!function_exists( 'evolution_woocommerce_checkout_fields' ) ) :
/**
* Kommentarfeld auf Kasse Seite entfernen
*/
function evolution_woocommerce_checkout_fields( $fields ) {
unset($fields['order']['order_comments']);
return $fields;
}
add_filter( 'woocommerce_checkout_fields' , 'evolution_woocommerce_checkout_fields' );
endif;