adjust visibility of DiscoveryNodes.Delta constructor

It can be private as it gets called by DiscoveryNodes#delta method, which is supposed to be the only way to create a Delta

Author: javanna

core/src/main/java/org/elasticsearch/cluster/node/DiscoveryNodes.java

@@ -396,7 +396,7 @@ public static class Delta {
         private final List<DiscoveryNode> removed;
         private final List<DiscoveryNode> added;
 
-        public Delta(@Nullable DiscoveryNode previousMasterNode, @Nullable DiscoveryNode newMasterNode, String localNodeId,
+        private Delta(@Nullable DiscoveryNode previousMasterNode, @Nullable DiscoveryNode newMasterNode, String localNodeId,
                      List<DiscoveryNode> removed, List<DiscoveryNode> added) {
             this.previousMasterNode = previousMasterNode;
             this.newMasterNode = newMasterNode;