Custom Walker for WordPress. Removes default classes but keeps any custom classes

custom-walker.php

// Custom walker. Removes default classes but keeps any custom classes

    class custom_walker extends Walker_Nav_Menu {
        function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
            $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

            $class_names = $value = '';

            $class_names = join( ' ', apply_filters( 
                'nav_menu_css_class', (array)get_post_meta( $item->ID, '_menu_item_classes', true ) ) 
            );

            $output .= $indent . '';

            $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
            $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
            $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
            $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
            $attributes .= ! empty( $item->classes )    ? ' class="'  . esc_attr( $class_names      ) .'"' : '';

            $item_output = $args->before;
            $item_output .= '<a'. $attributes .'>';
            $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
            $item_output .= '</a>';
            $item_output .= $args->after;

            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }

        function end_el( &$output, $item, $depth = 0, $args = array() ) {
            $output .= "\n";
        }
    }