[SPARK-16440][MLLIB] Undeleted broadcast variables in Word2Vec causing OoM for long runs

## What changes were proposed in this pull request?

Unpersist broadcasted vars in Word2Vec.fit for more timely / reliable resource cleanup

## How was this patch tested?

Jenkins tests

Author: Sean Owen <sowen@cloudera.com>

Closes #14153 from srowen/SPARK-16440.

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

@@ -434,6 +434,9 @@ class Word2Vec extends Serializable with Logging {
       bcSyn1Global.unpersist(false)
     }
     newSentences.unpersist()
+    expTable.unpersist()
+    bcVocab.unpersist()
+    bcVocabHash.unpersist()
 
     val wordArray = vocab.map(_.word)
     new Word2VecModel(wordArray.zipWithIndex.toMap, syn0Global)