[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.