remove unused import.

Author: hfutatzhanghb

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/StripeReader.java

@@ -38,9 +38,7 @@
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.Map;
-import java.util.Set;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CompletionService;
 import java.util.concurrent.ExecutorCompletionService;