MINOR: Remove `IndexTemplateFilter` (#32841)

* This isn't used anywhere anymore ever since `00c123b59f8ba11eb260e6b70acf7be80bccc949` and `dc166c5dc6bcf4abb7f25c6f4143f07d8176333d`

server/src/main/java/org/elasticsearch/cluster/metadata/IndexTemplateFilter.java

@@ -1,34 +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.cluster.metadata;
-
-import org.elasticsearch.action.admin.indices.create.CreateIndexClusterStateUpdateRequest;
-
-/**
- * Enables filtering the index templates that will be applied for an index, per create index request.
- */
-public interface IndexTemplateFilter {
-
-    /**
-     * @return  {@code true} if the given template should be applied on the newly created index,
-     *          {@code false} otherwise.
-     */
-    boolean apply(CreateIndexClusterStateUpdateRequest request, IndexTemplateMetaData template);
-}