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.Runtime/ServiceCollectionExtensions.cs

@@ -1,6 +1,4 @@
-using System.Collections.Immutable;
 using Akka.Actor;
-using Akka.Cluster.Tools.Client;
 using Akka.Event;
 using Akka.Hosting;
 using Microsoft.EntityFrameworkCore;
@@ -433,10 +431,9 @@ public static class ServiceCollectionExtensions
                 NodeCommunicationActor.Props(ackSender),
                 MeshPaths.NodeCommunicationName);
 
-            // Registered with the receptionist in ALL modes: under Dps and Grpc nothing dials it, and
-            // an idle registration costs nothing — whereas registering only under ClusterClient would
-            // make flipping the flag require a node restart before central could reach it.
-            ClusterClientReceptionist.Get(system).RegisterService(nodeComm);
+            // The registry entry is the whole reachability story: NodeCommandGrpcService resolves
+            // this key to hand an inbound command over. Nothing registers with a receptionist any
+            // more — there is no Akka association from central into this mesh to register into.
             registry.Register<NodeCommunicationActorKey>(nodeComm);
 
             // Continuous-historization recorder — gated on ContinuousHistorization:Enabled AND the