update individual members of Working Group

Created by: mojavelinux

Although Robert is a committer on the AsciiDoc Language project, and we very much appreciate his participation in this group, technically he's not a ratified member of the Working Group Steering Committee itself. And since this seems to be for ratified members only, we need to remove him. However, he will be included on the committers page for the AsciiDoc Language project once that site is up.