[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

mllib/src/main/scala/org/apache/spark/mllib/feature/Word2Vec.scala

@@ -434,15 +434,6 @@ class Word2VecModel private[mllib] (
     }
   }
   
-  /**
-   * Transforms an RDD to its vector representation
-   * @param dataset a an RDD of words 
-   * @return RDD of vector representation 
-   */
-  def transform(dataset: RDD[String]): RDD[Vector] = {
-    dataset.map(word => transform(word))
-  }
-  
   /**
    * Find synonyms of a word
    * @param word a word