[SPARK-12096][MLLIB] remove the old constraint in word2vec

jira: https://issues.apache.org/jira/browse/SPARK-12096

word2vec now can handle much bigger vocabulary.
The old constraint vocabSize.toLong * vectorSize < Ine.max / 8 should be removed.

new constraint is vocabSize.toLong * vectorSize < max array length (usually a little less than Int.MaxValue)

I tested with vocabsize over 18M and vectorsize = 100.

srowen jkbradley Sorry to miss this in last PR. I was reminded today.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #10103 from hhbyyh/w2vCapacity.