refactor(event-subscriber): drop the hoofdObject fallback (refs #153)

For a `resource: object` notification Objecten sends the object as both hoofdObject and
resourceUrl — the object *is* the main resource — so `HoofdObject ?? ResourceUrl` was a
branch that can never take its left side and that no test could distinguish. It came across
from the zaken path, where hoofdObject genuinely differed (the zaak behind a status).

Tests unchanged and green.

services/event-subscriber/EventSubscriber.Tests/NotificationProjectorTests.cs

@@ -22,7 +22,7 @@ public sealed class NotificationProjectorTests
     private Notification RecordWritten(string actie = "create", string url = ObjectUrl, string status = RegistrationStatus.Ingediend, string zaakId = ZaakId)
     {
         _acl.Records[url] = new RegisterRecord(zaakId, status, "REG-2026-0001");
-        return new Notification("objecten", "object", actie, new Uri(url), new Uri(url));
+        return new Notification("objecten", "object", actie, new Uri(url));
     }
 
     [Fact]