Removes display_element and updates $args->has_children to $this->has_children

`wp-bootstrap-navwalker` still works fine with Wordpress 4.0, but it has redundant code now that Wordpress 4.0 has updated the `class-wp-walker` and added a `$has_children` property. So, it's not necessary to hack the display_element function anymore. This commit removes `display_element` and updates `$args->has_children` to `$this->has_children`.