Remove unused interface InitialStateDiscoveryListener (#21115)

core/src/main/java/org/elasticsearch/discovery/InitialStateDiscoveryListener.java

@@ -1,33 +0,0 @@
-/*
- * Licensed to Elasticsearch under one or more contributor
- * license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright
- * ownership. Elasticsearch licenses this file to you under
- * the Apache License, Version 2.0 (the "License"); you may
- * not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing,
- * software distributed under the License is distributed on an
- * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
- * KIND, either express or implied.  See the License for the
- * specific language governing permissions and limitations
- * under the License.
- */
-
-package org.elasticsearch.discovery;
-
-/**
- * A listener that should be called by the {@link org.elasticsearch.discovery.Discovery} component
- * when the first valid initial cluster state has been submitted and processed by the cluster service.
- * <p>
- * Note, this listener should be registered with the discovery service before it has started.
- *
- *
- */
-public interface InitialStateDiscoveryListener {
-
-    void initialStateProcessed();
-}