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.

lib/build-clan/module.nix

@@ -94,9 +94,6 @@ let
               # Inherited from clan wide settings
               # TODO: remove these
 
-              # Machine specific settings
-              # clan.core.settings.machine.name = name;
-
               networking.hostName = lib.mkDefault name;
 
               # For vars we need to override the system so we run vars