It won't be in the header.
That "About Us" is after the breadcrumbs, and equivalent to the category name on a product page. I don't know the file offhand, but it will be a content/body tpl_ file, I suspect. It is possible that it could be removed by editing a define in a language file, such as ABOUT_US_HEADING or some such.
Look around and let us know what you find.