move note about removal of InitialClusterStateCustomSupplier to top-level class

Author: ywelsch

server/src/main/java/org/elasticsearch/plugins/ClusterPlugin.java

@@ -70,6 +70,8 @@ default void onNodeStarted() {
      * Returns a map of {@link ClusterState.Custom} supplier that should be invoked to initialize the initial clusterstate.
      * This allows custom clusterstate extensions to be always present and prevents invariants where clusterstates are published
      * but customs are not initialized.
+     *
+     * TODO: Remove this whole concept of InitialClusterStateCustomSupplier, it's not used anymore
      */
     default Map<String, Supplier<ClusterState.Custom>> getInitialClusterStateCustomSupplier() { return Collections.emptyMap(); }
 }

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/Security.java

@@ -903,12 +903,6 @@ public UnaryOperator<Map<String, IndexTemplateMetaData>> getIndexTemplateMetaDat
         };
     }
 
-    @Override
-    public Map<String, Supplier<ClusterState.Custom>> getInitialClusterStateCustomSupplier() {
-        // TODO: Remove this whole concept of InitialClusterStateCustomSupplier
-        return Collections.emptyMap();
-    }
-
     @Override
     public Function<String, Predicate<String>> getFieldFilter() {
         if (enabled) {