nixos/tests/containers: Remove unused module arg

Just removing the system argument because it doesn't exist (it's
actually config.nixpkgs.system, which we're already using). We won't get
an error anyway if we're not actually using it, so this is just an
aesthetics fix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/tests/containers-imperative.nix

@@ -7,7 +7,7 @@ import ./make-test.nix ({ pkgs, ...} : {
   };
 
   machine =
-    { config, pkgs, system, lib, ... }:
+    { config, pkgs, lib, ... }:
     { imports = [ ../modules/installer/cd-dvd/channel.nix ];
       virtualisation.writableStore = true;
       virtualisation.memorySize = 768;