Remove unused input and staff group

Signed-off-by: Benoit Donneaux <benoit@leastauthority.com>

Author: btlogy

nix/common/users.nix

@@ -1,8 +1,7 @@
 # This should be applied on all hosts
-{ config, lib, pkgs, ... }: {
+{ config, pkgs, ... }: {
   # always use only the configured users
   users.mutableUsers = false;
-  users.groups.staff = {};
   users.users = {
     root = {
       openssh.authorizedKeys.keys = [