refactor(registry): simplify stable uid validation message

registry/stable-uid.nix

@@ -1,7 +1,6 @@
 { lib, ... }:
 let
   inherit (lib)
-    concatStringsSep
     foldl'
     groupBy
     unique
@@ -25,8 +24,6 @@ let
         acc
     ) [ ] (builtins.attrNames byUid);
 
-  renderUidConflict = conflict:
-    "${toString conflict.uid} is assigned to ${concatStringsSep ", " conflict.serviceNames}";
 in
 {
   # Next stable UID: 20003
@@ -41,9 +38,7 @@ in
     [
       {
         assertion = uidConflicts == [ ];
-        message = ''
-          Duplicate stable UID allocation(s): ${concatStringsSep "\n" (map renderUidConflict uidConflicts)}
-        '';
+        message = "Stable UID allocations must be unique.";
       }
     ];
 }