Remove faulty check for 64-bit macOS

This check doesn't work and isn't needed. On 64-bit macOS, configure
output says:

checking for 64-bit Apple OSX... no

When it was first added in af4e52c, it checked whether "host" matched
"x86_64-*-darwin*" which would have been true on x86_64 Macs (but there
are other Macs with 64-bit processors: current Macs use arm64 processors
and Macs of decades past used ppc64 processors). Then in 2f11fb0 it was
changed to check whether "host_os" matched "darwin*" which would have
been true on all Macs, both 64-bit and 32-bit. Finally in d9f59fd it was
changed to check if "host_os" matches "x86_64-*darwin*" which will never
be true. Nothing based on this check is needed so the check can be
removed.

configure.ac

@@ -174,18 +174,6 @@ esac
 AC_MSG_RESULT([$cygwin])
 AM_CONDITIONAL(OS_CYGWIN, test "x$cygwin" = "xyes")
 
-AC_MSG_CHECKING([for 64-bit Apple OSX])
-case "$host_os" in
-  x86_64-*darwin*)
-    apple_osx=yes
-    ;;
-  *)
-    apple_osx=no
-    ;;
-esac
-AC_MSG_RESULT([$apple_osx])
-AM_CONDITIONAL(OS_X, test "x$apple_osx" = "xyes")
-
 AC_MSG_CHECKING([for NetBSD])
 case "$host_os" in
   *netbsd*)