YARN-11180. Fix CheckStyle.

Author: slfan1989

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-router/src/main/java/org/apache/hadoop/yarn/server/router/clientrm/FederationClientInterceptor.java

@@ -32,7 +32,6 @@
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ConcurrentHashMap;
-import java.util.Set;
 
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
@@ -692,7 +691,6 @@ public GetClusterMetricsResponse getClusterMetrics(
       RouterServerUtil.logAndThrowException("Missing getClusterMetrics request.", null);
     }
     long startTime = clock.getTime();
-    Map<SubClusterId, SubClusterInfo> subClusters = federationFacade.getSubClusters(true);
     ClientMethod remoteMethod = new ClientMethod("getClusterMetrics",
         new Class[] {GetClusterMetricsRequest.class}, new Object[] {request});
     Collection<GetClusterMetricsResponse> clusterMetrics = null;