simplemenu always changes css and js query strings resulting in huge bandwidth overhead

In _simplemenu_add_css() the call to variable_set('simplemenu_css_filename', $output_filename) is somehow commented.

Consequence of this is the fact that _drupal_flush_css_js() is called on each page request, this produce a new query string attached to css and js files making impossible for browsers/proxies any kind of caching.

This is critical as it would result in a huge bandwidth overhead because at every request browsers will download whole css and js files again and again.

Attached patch fixes this.