I've changed my mind about what the IP version selection options in

the config should do when IPv6 is configured out. They shouldn't sit
there looking silly with only `Auto' and `IPv4' settings: they
should instead be completely absent. I had thought the former was
acceptable since IPv4-only was a configuration that people should
only be using if their compilers didn't support IPv6, but now it
occurs to me that ports to fundamentally non-IPv6-supporting
platforms are not implausible, and on such ports the presence of a
vestigial config option under the _standard_ build conditions would
be a low-quality solution.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5084 cda61777-01e9-0310-a592-d414129be87e