Remove Sean, Winnie, and Annabel from Plan

@annabeldunstone please let me know if you'd like to be in future Plan retros; I think this is the right time to remove you?

@donaldcook I was in there anyway so I added Simon and removed Winnie. You'll also need to give Simon access to https://gitlab.com/gl-retrospectives/plan, if he doesn't have it already. (I can't give access any more: https://gitlab.com/gitlab-org/async-retrospectives/issues/28)