De-support SCO OpenServer/SCO UnixWare?

Currently, we have the following OS-specific configure templates in
src/template:

aix
cygwin
darwin
freebsd
hpux
linux
netbsd
openbsd
sco
solaris
unixware
win32

There is at least one active buildfarm critter testing each of these
except "sco" and "unixware".  I can't find any evidence in the buildfarm
database that we've *ever* had a SCO OpenServer buildfarm member.  There
were a couple of UnixWare members, but the last one was last heard from
on 2012-10-30 (around when 9.2 was released).  Mentions of these platforms
in the commit logs thin out to about nothing after 2008.

I think we should stop supporting those two platforms; to wit, we should
remove those template files and remove all supporting code and
documentation references to those platforms.  We can always put that stuff
back if someone shows up who's got enough interest to run a buildfarm
member.  But right now I'd say our support for those platforms is likely
to be hopelessly bit-rotted anyway, and it's more than likely that no one
cares.
        regards, tom lane