Remove PrevWordsInfo.getBigramsIteratorForPrediction().

This changes the behavior of predictions for some rare cases.
For example, let's consider the case of the prev word is
"ABC", "ABC" and "abc" is in the dict, "ABC" doesn't have
any bigrams, and "abc" has bigrams.
Without this change, "abc"'s bigrams are used for
prediction. But, with this change, "abc"'s bigrams are not
used for prediction. This strategy is the same as the the
current way of bigram probability computation during
traversal (for suggestion).

Bug: 14425059
Change-Id: I857ff39aa70aea265efa651451a63b759bb47d48