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
bcc8b14ef6