fix unused import.

Author: hfutatzhanghb

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/router/async/RouterAsyncRpcClient.java

@@ -37,9 +37,7 @@
 import org.apache.hadoop.hdfs.server.federation.router.async.utils.ApplyFunction;
 import org.apache.hadoop.hdfs.server.federation.router.async.utils.AsyncApplyFunction;
 import org.apache.hadoop.hdfs.server.federation.router.async.utils.AsyncCatchFunction;
-import org.apache.hadoop.ipc.CallerContext;
 import org.apache.hadoop.ipc.Client;
-import org.apache.hadoop.ipc.Server;
 import org.apache.hadoop.ipc.StandbyException;
 import org.apache.hadoop.security.UserGroupInformation;
 import org.slf4j.Logger;