Override core's system.menus.css since we already override the styles

In Drupal 7, the system.menus.css stylesheet is more-focused version of the Drupal 6 system-menus.css stylesheet. In Drupal 6, it was a difficult stylesheet to override because it included a lot of admin-specific styles.

In Drupal 7, the only admin-specific style is td.menu-disabled. I think that's a core bug, btw.

If we look at what's left in D7's system-menus.css we find:

• Menu tree styling (Zen has been tweaking this for a long time.)
• Tab styling (Zen has its own.)
• Breadcrumb styling (Zen has its own.)
• Inline link styling (Zen has its own.)
• Active link styling (Zen has a copy in its navigation.css.)

I'm trying to re-do the tab styling (originally written in Zen for Drupal 5) over in #1249790: Convert tabs from old sliding-door CSS technique to CSS3 and I'm realizing how annoying it is to override all the default tab styling when it would be much simpler to just remove it.