Remove redundant and incorrect default definition of AF_INET6. AF_INET6
is defined in sys/socket.h where it's defined as 28.

A bit of trivia: On NetBSD AF_INET6 is defined as 24. On Solaris it is
defined as 26. This is probably why Darren defaulted to 26, because
ipfilter was originally written for SunOS 4 and Solaris many moons ago.

MFC after: 2 weeks