[jvm-packages] removed old group_data from spark api (dmlc#3451)

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/params/LearningTaskParams.scala

@@ -55,13 +55,6 @@ private[spark] trait LearningTaskParams extends Params {
 
   final def getEvalMetric: String = $(evalMetric)
 
-  /**
-    * group data specify each group sizes for ranking task. To correspond to partition of
-    * training data, it is nested.
-    */
-  final val groupData = new GroupDataParam(this, "groupData", "group data specify each group " +
-    "size for ranking task. To correspond to partition of training data, it is nested.")
-
   /**
    * Fraction of training points to use for testing.
    */
@@ -82,7 +75,7 @@ private[spark] trait LearningTaskParams extends Params {
 
   final def getNumEarlyStoppingRounds: Int = $(numEarlyStoppingRounds)
 
-  setDefault(objective -> "reg:linear", baseScore -> 0.5, groupData -> null,
+  setDefault(objective -> "reg:linear", baseScore -> 0.5,
     trainTestRatio -> 1.0, numEarlyStoppingRounds -> 0)
 }