Unregister Nav Menu from Child-Theme

I like to use the Starkers Theme and build child-theme.

So, I`d like to remove the this primary menu

Starkers (functions.php)

function starkers_setup() {
// This theme uses wp_nav_menu() in one location.
register_nav_menus( array(
'primary' => __( 'Primary Navigation', 'starkers' ),
) );


}
endif;

My child-theme functions.php looks like this:

unregister_nav_menu( array(
'primary' => __( 'Primary Navigation', 'starkers' ),
));

// my new nav
register_nav_menus(array(
'primary_navigation' => __('Main', 'starkers'),
'utility_navigation' => __('Meta', 'starkers')
));

But it doesn´t work for me.

What´s wrong?
Thanks