linux_*_hardened: don't set X86_X32

As far as I can tell, this has never defaulted to on upstream, and our
common kernel configuration doesn't turn it on, so the attack surface
reduction here is somewhat homeopathic.

pkgs/os-specific/linux/kernel/hardened-config.nix

@@ -19,8 +19,6 @@ assert (versionAtLeast version "4.9");
 optionalAttrs (stdenv.hostPlatform.platform.kernelArch == "x86_64") {
   DEFAULT_MMAP_MIN_ADDR = freeform "65536";  # Prevent allocation of first 64K of memory
 
-  # Reduce attack surface by disabling X32
-  X86_X32            = no;
   # Note: this config depends on EXPERT y and so will not take effect, hence
   # it is left "optional" for now.
   MODIFY_LDT_SYSCALL = option no;