'block_callback' in menu_router table seems to have no purpose

The block_callback field in the {menu_router} table seems to be a curious leftover from pre-D6 at least.

It's only used in system_main_admin_page():

      if ($item['block_callback'] && function_exists($item['block_callback'])) {
        $function = $item['block_callback'];
        $block['content'] .= $function();
      }

There's not a single item in my menu_router table that has this set. On D6, there's only one, and it's not even relevant to system_main_admin_page()!

I reckon we can get rid of this.