Use this anywhere in the css and it should work.
#nav_menu {
display: none !important;
}
or remove this from the header.php and it should work
<?php /* Our navigation menu. If one isn’t filled out, wp_nav_menu falls back to wp_page_menu. The menu assiged to the primary position is the one used. If none is assigned, the menu with the lowest ID is used. */ ?>
<?php wp_nav_menu( array( ‘container_class’ => ‘menu-header’, ‘theme_location’ => ‘primary’ ) ); ?>
Note I haven’t tested either so if it doesn’t work let me know and I will check.
