tests/default.nix: removed system.stateVersion declaration

Having the declaration of system.stateVersion just adds more
maintainance for devs. For example, changing the nixpkgs value in
flake.nix requires a change of system.stateVersion in tests/default.nix.

flake.nix

@@ -1,7 +1,7 @@
 {
   description = "An Invoice Ninja package and module.";
 
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
 
   outputs =
     { self, nixpkgs, nix, }:

tests/default.nix

@@ -6,8 +6,6 @@
     ../nixos-module/invoiceninja.nix
   ];
 
-  system.stateVersion = "25.11";
-
   nixpkgs.config.allowUnfree = true;
 
   users.users.test = {