YARN-11012. Fix checkstyle

Author: luoyuan

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-router/src/test/java/org/apache/hadoop/yarn/server/router/TestRouter.java

@@ -19,7 +19,6 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
 import org.apache.hadoop.conf.Configuration;