remove default for clan.core.settings.machine.name and make readOnly

There is not reason that there needs to be a default. Machines are usually defined as an attrset, so there is no reason to have a default.
Also make this readOnly as we never want to override it.

nixosModules/clanCore/metadata.nix

@@ -117,7 +117,7 @@ in
               options = {
                 name = mkOption {
                   type = types.str;
-                  default = "nixos";
+                  readOnly = true;
                   description = ''
                     the name of the machine
                   '';