Remove default value for nixpkgs.system

Using "builtins.currentSystem" doesn't work in pure evaluation mode,
and even when it's explicitly set (which it always is, in
nixos/lib/eval-config.nix), it breaks manual generation because the
manual tries to render the default value.

nixos/modules/misc/nixpkgs.nix

···
     system = mkOption {
       type = types.str;
       example = "i686-linux";
-      default = { system = builtins.currentSystem; };
       description = ''
         Specifies the Nix platform type on which NixOS should be built.
         It is better to specify <code>nixpkgs.localSystem</code> instead.