linux: remove remnants of grsecurity

Nixpkgs hasn't supported grsecurity kernels since 2017, so unless
anybody is manually enabling the grsecurity feature to make these
small kernel tweaks this is dead code.

This means we don't actually support any "features" in the kernel
common-config any more, but I've left the argument there because it's
conceivable we could have some again in future.

pkgs/os-specific/linux/kernel/generic.nix

@@ -72,7 +72,6 @@ let
     efiBootStub = true;
     needsCifsUtils = true;
     netfilterRPFilter = true;
-    grsecurity = false;
     ia32Emulation = true;
   } // features) kernelPatches;