HDFS-15309. Remove redundant String.valueOf method on ExtendedBlockId.java. Contributed by bianqi.

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

@@ -76,6 +76,6 @@ final public class ExtendedBlockId {
 
   @Override
   public String toString() {
-    return String.valueOf(blockId) + "_" + bpId;
+    return blockId + "_" + bpId;
   }
 }