Remove useless symbol from Makefile.global.

I added HAVE_IPV6 to Makefile.global way back in commit 7703e55c3
so that we could transmit its value to the shell-script version of
initdb.  Since initdb was rewritten in C, it's been finding that
out from pg_config.h instead, so this is useless.  Keeping it here
just wastes configure and make cycles, plus it's a potential
two-sources-of-truth problem.

configure

@@ -650,7 +650,6 @@ PG_CRC32C_OBJS
 CFLAGS_ARMV8_CRC32C
 CFLAGS_SSE42
 have_win32_dbghelp
-HAVE_IPV6
 LIBOBJS
 UUID_LIBS
 LDAP_LIBS_BE
@@ -15347,7 +15346,6 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
-HAVE_IPV6=no
 ac_fn_c_check_type "$LINENO" "struct sockaddr_in6" "ac_cv_type_struct_sockaddr_in6" "$ac_includes_default
 #include <netinet/in.h>
 "
@@ -15355,11 +15353,9 @@ if test "x$ac_cv_type_struct_sockaddr_in6" = xyes; then :
 
 $as_echo "#define HAVE_IPV6 1" >>confdefs.h
 
-         HAVE_IPV6=yes
 fi
 
 
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for PS_STRINGS" >&5
 $as_echo_n "checking for PS_STRINGS... " >&6; }
 if ${pgac_cv_var_PS_STRINGS+:} false; then :

configure.in

@@ -1652,14 +1652,11 @@ AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])
 
 AC_CHECK_DECLS([RTLD_GLOBAL, RTLD_NOW], [], [], [#include <dlfcn.h>])
 
-HAVE_IPV6=no
 AC_CHECK_TYPE([struct sockaddr_in6],
-        [AC_DEFINE(HAVE_IPV6, 1, [Define to 1 if you have support for IPv6.])
-         HAVE_IPV6=yes],
+        [AC_DEFINE(HAVE_IPV6, 1, [Define to 1 if you have support for IPv6.])],
         [],
 [$ac_includes_default
 #include <netinet/in.h>])
-AC_SUBST(HAVE_IPV6)
 
 AC_CACHE_CHECK([for PS_STRINGS], [pgac_cv_var_PS_STRINGS],
 [AC_LINK_IFELSE([AC_LANG_PROGRAM(

src/Makefile.global.in

@@ -497,9 +497,6 @@ host_tuple = @host@
 host_os = @host_os@
 host_cpu = @host_cpu@
 
-# Make HAVE_IPV6 available for initdb script creation
-HAVE_IPV6= @HAVE_IPV6@
-
 # This is mainly for use on FreeBSD, where we have both a.out and elf
 # systems now.  May be applicable to other systems to?
 ELF_SYSTEM= @ELF_SYS@