net/unison2.51: Return to installing as "unison".

The short summary is that there is now one unison package which
installs a binary named "unison".

unison has multiple branches, but upstream only maintains 2.51.
Because the wire protocol also depends (sometimes, such as in the case
of 4.08) on the ocaml version because of serialization, older unison
built with 4.08 will not interoperate with those older versions built
with earlier ocaml.  Therefore older versions in pkgsrc have no
utility, and have been removed.  (In addition, zero people commented
about using them.)

Until recently, this package used to install as "unison", as opposed
to to the 2.32, 2.40, and 2.48 packages which installed as
"unison-2.xx", because 2.51 is the standard approach and the others
were an accomodation for those interoperating with systems with old
software.  Now, with only one unison package, this is
counterproductive.  (In addition, the change to install as 2.51 was
not discussed, was objected to, and no reply comments were received.)

(gdt)