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.

tests/acceptance/Steps/RegisterProjectieBijwerkenSteps.cs

@@ -28,7 +28,7 @@ public sealed class RegisterProjectieBijwerkenSteps
         // The ACL upserts one object per registration, so submit and approval share an object URL.
         var objectUrl = ObjectBase + id;
         _register.Records[objectUrl] = new RegisterRecord(id, status, "REG-" + id);
-        _notification = new Notification("objecten", "object", "create", new Uri(objectUrl), new Uri(objectUrl));
+        _notification = new Notification("objecten", "object", "create", new Uri(objectUrl));
     }
 
     [Given("the register notification is delivered to the event subscriber")]