remove diffutils from appveyor config entirely
remove diffutils from appveyor config entirely

rgrinberg/ocaml-conduit,djs55/ocaml-conduit,rgrinberg/ocaml-conduit,samoht/ocaml-conduit,avsm/ocaml-conduit,samoht/ocaml-conduit,djs55/ocaml-conduit,mirage/ocaml-conduit