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<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

//* Reduce the secondary navigation menu to one level depth
add_filter( 'wp_nav_menu_args', 'ns_secondary_menu_args' );
function ns_secondary_menu_args( $args ){

    if( 'secondary' != $args['theme_location'] )
    return $args;

    $args['depth'] = 1;
    return $args;

}

<!-- From No Sidebar Pro's functions.php -->