nvidia-x11: remove unused binding to kernelModVersion

pkgs/os-specific/linux/nvidia-x11/default.nix

@@ -13,8 +13,6 @@ let
   kernel = callPackage # a hacky way of extracting parameters from callPackage
     ({ kernel, libsOnly ? false }: if libsOnly then { } else kernel) { };
 
-  kernelModVersion = lib.versions.majorMinor kernel.modDirVersion;
-
   selectHighestVersion = a: b: if lib.versionOlder a.version b.version
     then b
     else a;