from time to time you need to completely change the way a menu markup is displayed and the walker class is a pain. you can just use this procedural function and add classes or markup you need

super-clean-wp-menu.php

function clean_custom_menus() {
	$menu_name = 'nav-primary'; // specify custom menu slug
	if (($locations = get_nav_menu_locations()) && isset($locations[$menu_name])) {
		$menu = wp_get_nav_menu_object($locations[$menu_name]);
		$menu_items = wp_get_nav_menu_items($menu->term_id);

		$menu_list = '<nav>';
		$menu_list .= '<ul>';
		foreach ((array) $menu_items as $key => $menu_item) {
			$title = $menu_item->title;
			$url = $menu_item->url;
			$menu_list .= '<li><a href="'. $url .'">'. $title .'</a></li>';
		}
		$menu_list .= '</ul>';
		$menu_list .= '</nav>';
	} else {
		// $menu_list = '<!-- no list defined -->';
	}
	echo $menu_list;
}

wp menu clean output

it will return something like this

<nav>
	<ul>
		<li><a href="http://example.com/">Home</a></li>
		<li><a href="http://example.com/demos/">Demos</a></li>
		<li><a href="http://example.com/downloads/">Downloads</a></li>
		<li><a href="http://example.com/docs/">Documentation</a></li>
	</ul>
</nav>