Revert "Potential thread concurrency issue with WordIterators."

This reverts commit d224c88c.

Based on Dianne's comments, this small optimization seems uncessarily
risky. I'll submit a new change where each SpellParser has its own
WordIterator to make this thread-safe.

Change-Id: Ic09fa656b00d284536e58f4cc7d26d5e26c3f3cf