[FLINK] remove nWorker from API

jvm-packages/xgboost4j-flink/src/main/scala/ml/dmlc/xgboost4j/scala/flink/XGBoost.scala

@@ -82,9 +82,9 @@ object XGBoost {
     * @param params The parameters to XGBoost.
     * @param round Number of rounds to train.
     */
-  def train(dtrain: DataSet[LabeledVector], params: Map[String, Any], round: Int, nWorkers: Int):
+  def train(dtrain: DataSet[LabeledVector], params: Map[String, Any], round: Int):
       XGBoostModel = {
-    val tracker = new RabitTracker(nWorkers)
+    val tracker = new RabitTracker(dtrain.getExecutionEnvironment.getParallelism)
     if (tracker.start()) {
       dtrain
         .mapPartition(new MapFunction(params, round, tracker.getWorkerEnvs))