Remove useless symbol from Makefile.global.

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.