Scripting: Remove native scripts (#24726)

Native scripts have been replaced in documentation by implementing
a ScriptEngine and they were deprecated in 5.5.0. This commit
removes the native script infrastructure for 6.0.

closes #19966

Author: rjernst

core/src/main/java/org/elasticsearch/plugins/ScriptPlugin.java

@@ -19,13 +19,9 @@
 package org.elasticsearch.plugins;
 
 import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.script.NativeScriptFactory;
 import org.elasticsearch.script.ScriptContext;
 import org.elasticsearch.script.ScriptEngine;
 
-import java.util.Collections;
-import java.util.List;
-
 /**
  * An additional extension point for {@link Plugin}s that extends Elasticsearch's scripting functionality.
  */
@@ -38,13 +34,6 @@ default ScriptEngine getScriptEngine(Settings settings) {
         return null;
     }
 
-    /**
-     * Returns a list of {@link NativeScriptFactory} instances.
-     */
-    default List<NativeScriptFactory> getNativeScripts() {
-        return Collections.emptyList();
-    }
-
     /**
      * Returns a {@link ScriptContext.Plugin} instance or <code>null</code> if this plugin doesn't add a new script context plugin
      */