[MLlib] Remove transform(dataset: RDD[String]) from Word2Vec public API

mengxr
Remove  transform(dataset: RDD[String]) from public API.

Author: Liquan Pei <liquanpei@gmail.com>

Closes #2010 from Ishiihara/Word2Vec-api and squashes the following commits:

17b1031 [Liquan Pei] remove transform(dataset: RDD[String]) from public API

(cherry picked from commit 9306b8c6c8c412b9d0d5cffb6bd7a87784f0f6bf)
Signed-off-by: Xiangrui Meng <meng@databricks.com>