nixos/nix-daemon: Move to services/system

It is now only about the system service.
Granted, it also installs the client package, but that could be
factored out later, with actual test to support such a new type of
configuration.

nixos/modules/module-list.nix

@@ -665,7 +665,6 @@
   ./services/misc/moonraker.nix
   ./services/misc/n8n.nix
   ./services/misc/nitter.nix
-  ./services/misc/nix-daemon.nix
   ./services/misc/nix-gc.nix
   ./services/misc/nix-optimise.nix
   ./services/misc/nix-ssh-serve.nix
@@ -1147,6 +1146,7 @@
   ./services/system/earlyoom.nix
   ./services/system/kerberos/default.nix
   ./services/system/localtimed.nix
+  ./services/system/nix-daemon.nix
   ./services/system/nscd.nix
   ./services/system/saslauthd.nix
   ./services/system/self-deploy.nix