Fix JDK7 compatibility in branch-2.

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/ha/HATestUtil.java

@@ -36,7 +36,6 @@ import java.util.List;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicLong;
-import java.util.concurrent.atomic.LongAccumulator;
 
 import com.google.common.base.Function;
 import com.google.common.base.Joiner;