Remove code to match IPv4 pg_hba.conf entries to IPv4-in-IPv6 addresses.

In investigating yesterday's crash report from Hugo Osvaldo Barrera, I only
looked back as far as commit f3aec2c7f5 where the breakage occurred
(which is why I thought the IPv4-in-IPv6 business was undocumented).  But
actually the logic dates back to commit 3c9bb8886d and was simply
broken by erroneous refactoring in the later commit.  A bit of archives
excavation shows that we added the whole business in response to a report
that some 2003-era Linux kernels would report IPv4 connections as having
IPv4-in-IPv6 addresses.  The fact that we've had no complaints since 9.0
seems to be sufficient confirmation that no modern kernels do that, so
let's just rip it all out rather than trying to fix it.

Do this in the back branches too, thus essentially deciding that our
effective behavior since 9.0 is correct.  If there are any platforms on
which the kernel reports IPv4-in-IPv6 addresses as such, yesterday's fix
would have made for a subtle and potentially security-sensitive change in
the effective meaning of IPv4 pg_hba.conf entries, which does not seem like
a good thing to do in minor releases.  So let's let the post-9.0 behavior
stand, and change the documentation to match it.

In passing, I failed to resist the temptation to wordsmith the description
of pg_hba.conf IPv4 and IPv6 address entries a bit.  A lot of this text
hasn't been touched since we were IPv4-only.

src/include/libpq/ip.h

@@ -46,11 +46,6 @@ extern int pg_range_sockaddr(const struct sockaddr_storage * addr,
 extern int pg_sockaddr_cidr_mask(struct sockaddr_storage * mask,
 					  char *numbits, int family);
 
-#ifdef HAVE_IPV6
-extern void pg_promote_v4_to_v6_addr(struct sockaddr_storage * addr);
-extern void pg_promote_v4_to_v6_mask(struct sockaddr_storage * addr);
-#endif
-
 extern int	pg_foreach_ifaddr(PgIfAddrCallback callback, void *cb_data);
 
 #endif   /* IP_H */