jool: removed kernel version assertion

I don't think this really makes sense, because upstream doesn't list any constraints on supported kernel versions, and worst case it'll just fail to build.

pkgs/os-specific/linux/jool/default.nix

@@ -1,7 +1,5 @@
 { stdenv, fetchFromGitHub, kernel }:
 
-assert stdenv.lib.versionOlder kernel.version "4.20";
-
 let
   sourceAttrs = (import ./source.nix) { inherit fetchFromGitHub; };
 in