remove li from wp_nav_menu, preserve classes, custom walker

walker.php

<?php 
// based on, fixed warnings http://www.designtoday.info/removing-li-menu-from-wordpress/

class Description_Walker extends Walker_Nav_Menu {
  function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
    $classes = empty($item->classes) ? array () : (array) $item->classes;
    $class_names = join(' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
    !empty ( $class_names ) and $class_names = ' class="'. esc_attr( $class_names ) . '"';
    $output .= "";
    $attributes  = '';
    !empty( $item->attr_title ) and $attributes .= ' title="'  . esc_attr( $item->attr_title ) .'"';
    !empty( $item->target ) and $attributes .= ' target="' . esc_attr( $item->target     ) .'"';
    !empty( $item->xfn ) and $attributes .= ' rel="'    . esc_attr( $item->xfn        ) .'"';
    !empty( $item->url ) and $attributes .= ' href="'   . esc_attr( $item->url        ) .'"';
    $title = apply_filters( 'the_title', $item->title, $item->ID );
    $item_output = $args->before
    . "<a $attributes $class_names>"
    . $args->link_before
    . $title
    . '</a>'
    . $args->link_after
    . $args->after;
    $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
  }
}

// usage
$menu = wp_nav_menu( array(
  'theme_location' => 'top_menu', // we've registered a theme location in functions.php
  'container' => false, // this is usually a div outside the menu ul, we don't need it
  'items_wrap' => '<nav id="%1$s" class="%2$s">%3$s</nav>', // replacing the ul with nav
  'echo' => false, // don't display it just yet, instead we're storing it in the variable $cleanermenu
  'walker' => new Description_Walker,
  'menu_class' => "Menu"
));
?>