Как удалить sitemap из WMT?

Susan Moskwa:
The best way to stop Googlebot from crawling URLs that it has discovered in the past is to make those URLs (such as your old Sitemaps) 404. After seeing that a URL repeatedly 404s, we stop crawling it. And after we stop crawling a Sitemap, it should drop out of your "All Sitemaps" tab.

Alternatively, if you don't want to deal with the fact that we still know about these URLs, just don't look at the "All" tab. No one is "forcing" you to maintain anything; we're just letting you know that this is a URL Googlebot discovered at some point.