HDFS-5985. SimulatedFSDataset#disableAndPurgeTrashStorage should not throw UnsupportedOperationException. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-5535@1570363 13f79535-47bb-0310-9956-ffa450edef68