nixos/tests: remove minimal-kernel module

It's not used, doesn't build, and seems like the only reason to have
`manualConfig` take `stdenv` as an argument.

nixos/lib/testing/legacy.nix

@@ -3,9 +3,10 @@ let
   inherit (lib) mkIf mkOption types;
 in
 {
-  # This needs options.warnings, which we don't have (yet?).
+  # This needs options.warnings and options.assertions, which we don't have (yet?).
   # imports = [
   #   (lib.mkRenamedOptionModule [ "machine" ] [ "nodes" "machine" ])
+  #   (lib.mkRemovedOptionModule [ "minimal" ] "The minimal kernel module was removed as it was broken and not used any more in nixpkgs.")
   # ];
 
   options = {