nvidia-x11: remove unused binding to kernelModVersion

  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;