net/unison2.40: Remove

As proposed on pkgsrc-users with no comments.  Note that unison has
multiple versions only to deal with wire protocol breaks that happen
every version; without that there would just be a single recent unison
version.

In addition to being very old and unused, I believe that 2.40 from
pkgsrc cannot work to interoperate with other systems that still have
2.40, because pkgsrc's ocaml version has an incompatible serialization
format, and the unison wire protocol is effectively the combination of
the unison version and the ocaml serialization version.

(Separate from 2.32 removal to ease separate reverting, should it be
necessary.)


(gdt)