[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