[SPARK-4998][MLlib]delete the "train" function

To make the functions with the same in "object" effective, specially when using java reflection.
As the "train" function defined in "class DecisionTree" will hide the functions with the same name in "object DecisionTree".

JIRA[SPARK-4998]

Author: Liu Jiongzhou <ljzzju@163.com>

Closes #3836 from ljzzju/master and squashes the following commits:

4e13133 [Liu Jiongzhou] [MLlib]delete the "train" function

mllib/src/main/scala/org/apache/spark/mllib/tree/DecisionTree.scala

@@ -64,13 +64,6 @@ class DecisionTree (private val strategy: Strategy) extends Serializable with Lo
     val rfModel = rf.run(input)
     rfModel.trees(0)
   }
-
-  /**
-   * Trains a decision tree model over an RDD. This is deprecated because it hides the static
-   * methods with the same name in Java.
-   */
-  @deprecated("Please use DecisionTree.run instead.", "1.2.0")
-  def train(input: RDD[LabeledPoint]): DecisionTreeModel = run(input)
 }
 
 object DecisionTree extends Serializable with Logging {