A Cre8asite Forums thread, one of the last non-Google forums to still have a webmaster related Googler stop by, has discussion around removing URLs from your XML Sitemap file. There is a myth that if you are missing URLs or remove URLs from your XML Sitemap file, that Google would remove and not list those URLs in their index. That is not true and there is nothing more false about that.
An XML Sitemap file is just an extra tool for Google to discover your content, but it is not necessary for Google to index your content.
Googler, JohnMu, said:
We use Sitemap files for discovering new URLs and recognizing when existing ones have changed; we don't remove URLs from the index that aren't in a Sitemap file.
But if you do remove a URL from the Sitemap file, Google may take that as a sign to check the URL and see if it is still there. If the URL is still there, then no worries. John added:
In the worst case we'll just recrawl it to see if it has changed.
Forum discussion at Cre8asite Forums.