Remove dead setenv, unsetenv replacement code.

setenv() and unsetenv() are in SUSv3 and targeted Unix systems have
them.  We still need special code for these on Windows, but that doesn't
require a configure probe.

This marks the first time we require a SUSv3 (POSIX.1-2001) facility
(rather than SUSv2).  The replacement code removed here was not needed
on any targeted system or any known non-EOL'd Unix system, and was
therefore dead and untested.

No need for vestigial HAVE_SETENV and HAVE_UNSETENV macros, because we
provide a replacement for Windows, and we didn't previously test the
macros.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Greg Stark <stark@mit.edu>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

configure.ac

@@ -1898,19 +1898,6 @@ if test "$PORTNAME" = "win32" -o "$PORTNAME" = "cygwin"; then
 	AC_MSG_NOTICE([On $host_os we will use our strtof wrapper.])
 fi
 
-case $host_os in
-        # Windows uses a specialised env handler
-        mingw*)
-                AC_DEFINE(HAVE_SETENV, 1, [Define to 1 because replacement version used.])
-                AC_DEFINE(HAVE_UNSETENV, 1, [Define to 1 because replacement version used.])
-                ac_cv_func_setenv=yes
-                ac_cv_func_unsetenv=yes
-                ;;
-        *)
-                AC_REPLACE_FUNCS([setenv unsetenv])
-                ;;
-esac
-
 # System's version of getaddrinfo(), if any, may be used only if we found
 # a definition for struct addrinfo; see notes in src/include/getaddrinfo.h.
 # We use only our own getaddrinfo.c on Windows, but it's time to revisit that.