HDFS-16401.Remove the worthless DatasetVolumeChecker#numAsyncDatasetChecks. (apache#3838)

Author: jianghuazhu

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/checker/DatasetVolumeChecker.java

@@ -77,7 +77,6 @@ public class DatasetVolumeChecker {
 
   private final AtomicLong numVolumeChecks = new AtomicLong(0);
   private final AtomicLong numSyncDatasetChecks = new AtomicLong(0);
-  private final AtomicLong numAsyncDatasetChecks = new AtomicLong(0);
   private final AtomicLong numSkippedChecks = new AtomicLong(0);
 
   /**