remove used statement.

Author: hfutatzhanghb

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java

@@ -138,7 +138,6 @@
 class FsDatasetImpl implements FsDatasetSpi<FsVolumeImpl> {
   static final Logger LOG = LoggerFactory.getLogger(FsDatasetImpl.class);
   private final static boolean isNativeIOAvailable;
-  private static final String SEP = System.getProperty("file.separator");
   private Timer timer;
   static {
     isNativeIOAvailable = NativeIO.isAvailable();