GN: Remove Value::RecursivelySetOrigin.

This method has been unused for about four months:
  https://codereview.chromium.org/223783005/

It is probably worthwhile to remove it, since it's dead and untested.

Review URL: https://codereview.chromium.org/445393004

git-svn-id: http://src.chromium.org/svn/trunk/src/tools/gn@288744 4ff67af0-8c30-449e-8e8b-ad334ec8d88c