Remove field no longer used after previous commit

Issue: SPR-11516

spring-test/src/main/java/org/springframework/test/web/servlet/DefaultMvcResult.java

@@ -15,7 +15,6 @@
  */
 package org.springframework.test.web.servlet;
 
-import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.springframework.mock.web.MockHttpServletRequest;
@@ -52,8 +51,6 @@ class DefaultMvcResult implements MvcResult {
 
 	private final AtomicReference<Object> asyncResult = new AtomicReference<Object>(RESULT_NONE);
 
-	private CountDownLatch asyncResultLatch;
-
 
 	/**
 	 * Create a new instance with the given request and response.