code: public static void deleteSegment(Olap olap, ApplicationDefinition appDef, String shard, String segment) {
          VDirectory appDir = olap.getRoot(appDef);
          VDirectory shardDir = appDir.getDirectory(shard);
          VDirectory segmentDir = shardDir.getDirectory(segment);
          segmentDir.delete();
      }
docs: delete a particular segment in the shard.