Remove configure probe for sockaddr_in6 and require AF_INET6.

Remove configure probe for sockaddr_in6 and require AF_INET6.

SUSv3 <netinet/in.h> defines struct sockaddr_in6, and all targeted Unix
systems have it.  Windows has it in <ws2ipdef.h>.  Remove the configure
probe, the macro and a small amount of dead code.

Also remove a mention of IPv6-less builds from the documentation, since
there aren't any.

This is similar to commits f5580882 and 077bf2f2 for Unix sockets.  Even
though AF_INET6 is an "optional" component of SUSv3, there are no known
modern operating system without it, and it seems even less likely to be
omitted from future systems than AF_UNIX.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com