refactor(mesh): step 3 — delete both ClusterClientReceptionist registrations

Central's CentralCommunicationActor and a node's NodeCommunicationActor no longer register
with a receptionist; the ActorRegistry entry each already carried is now the whole
reachability story, and it is what CentralCommandGrpcService / NodeCommandGrpcService
resolve to hand an inbound message over. Both `using Akka.Cluster.Tools.Client` directives
are gone with them.

src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/ServiceCollectionExtensions.cs

@@ -1,6 +1,5 @@
 using Akka.Actor;
 using Akka.Cluster.Hosting;
-using Akka.Cluster.Tools.Client;
 using Akka.Hosting;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
@@ -94,11 +93,9 @@ public static class ServiceCollectionExtensions
                     () => serviceProvider.GetService<IMeshPskProvider>()),
                 MeshPaths.CentralCommunicationName);
 
-            // Registered unconditionally, in both transport modes. Under Dps nothing dials it, and
-            // an idle registration costs nothing — whereas registering only under ClusterClient
-            // would mean flipping the flag on a running fleet requires a central restart before any
-            // node can ack, turning a config change back into a deployment.
-            ClusterClientReceptionist.Get(system).RegisterService(actor);
+            // The registry entry is the whole reachability story: CentralCommandGrpcService resolves
+            // this key to hand an inbound ack over. Nothing registers with a receptionist any more —
+            // there is no Akka association between central and a site mesh to register into.
             registry.Register<CentralCommunicationActorKey>(actor);
         });