Removes support for null TransportService in RemoteClusterService (#137939)

This change is a follow up to #137406, removing a null check
around the TransportService instance when registering the metric.
This was only needed for a few test cases in the
RemoteClusterServiceTests where a null instance was passed to the
constructor.  The change also cleans up the code for creating the
TransportService instances in the test, cutting down on the LOC.

Relates: #137406

Author: JeremyDahlgren

server/src/main/java/org/elasticsearch/transport/RemoteClusterService.java

@@ -102,11 +102,9 @@ public boolean isRemoteClusterServerEnabled() {
          *  the functionality to do it the right way is not yet ready -- replace this code when it's ready.
          */
         this.crossProjectEnabled = settings.getAsBoolean("serverless.cross_project.enabled", false);
-        if (transportService != null) {
-            transportService.getTelemetryProvider()
-                .getMeterRegistry()
-                .registerLongCounter(CONNECTION_ATTEMPT_FAILURES_COUNTER_NAME, "linked project connection attempt failure count", "count");
-        }
+        transportService.getTelemetryProvider()
+            .getMeterRegistry()
+            .registerLongCounter(CONNECTION_ATTEMPT_FAILURES_COUNTER_NAME, "linked project connection attempt failure count", "count");
     }
 
     public RemoteClusterCredentialsManager getRemoteClusterCredentialsManager() {