Simplify TransportMultiSearchActionTests (#48523)

The test doesn't seem to need the threadpool that is created and destroyed in
setup and teardown any longer, so it can be removed.

server/src/test/java/org/elasticsearch/action/search/TransportMultiSearchActionTests.java

@@ -39,12 +39,9 @@
 import org.elasticsearch.tasks.Task;
 import org.elasticsearch.tasks.TaskManager;
 import org.elasticsearch.test.ESTestCase;
-import org.elasticsearch.threadpool.TestThreadPool;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.Transport;
 import org.elasticsearch.transport.TransportService;
-import org.junit.After;
-import org.junit.Before;
 
 import java.util.Arrays;
 import java.util.Collections;
@@ -63,22 +60,6 @@
 
 public class TransportMultiSearchActionTests extends ESTestCase {
 
-    protected ThreadPool threadPool;
-
-    @Before
-    @Override
-    public void setUp() throws Exception {
-        super.setUp();
-        threadPool = new TestThreadPool(getTestName());
-    }
-
-    @After
-    @Override
-    public void tearDown() throws Exception {
-        threadPool.shutdown();
-        super.tearDown();
-    }
-
     public void testParentTaskId() throws Exception {
         // Initialize dependencies of TransportMultiSearchAction
         Settings settings = Settings.builder()