Remove sync from ConnectionTarget#toString (#128656)

When introduced in #39629 we needed to acquire `mutex` here because the
`toString()` result included a mutable field, but we dropped that field
in #77295. This commit removes the now-unnecessary synchronization.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/cluster/NodeConnectionsService.java

@@ -342,9 +342,7 @@ void disconnect() {
 
         @Override
         public String toString() {
-            synchronized (mutex) {
-                return "ConnectionTarget{" + "discoveryNode=" + discoveryNode + '}';
-            }
+            return "ConnectionTarget{discoveryNode=" + discoveryNode + '}';
         }
     }
 }