README: drop recommendation to delete entries
Some people are avoiding adding redirects here because they already updated the Vcs fields in the package and the README says it's not needed any more.
However, redirects are still needed because stable Debian releases still have the old Vcs entries. People still have the old repos in their browser history and bookmarks.
I'm not really aware of people removing old entries, so let's clean up the README to better reflect current practice.