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

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.

[originally from svn r5084]