nixos: change home-manager backup extension

Use 'hm-backup' globally and remove the per-user override.

nixos/ben.nix

@@ -1,6 +1,5 @@
 { pkgs, inputs, config, makeEnable, ... }:
 makeEnable config "myModules.ben" true {
-  home-manager.backupFileExtension = "backup"; # Add this line
   home-manager.users.ben = {
     programs.zsh = {
       enable = true;