Put back `remove_unsafe_private_use` to avoid an API change

I was removing this option prematurely, because the characters it
removes are no longer unsafe in up-to-date Python. However:

- Removing the option would be an API change that breaks compatibility
- Some versions of Linux distributions are still on Python 3.3.2, and
  will not update it without a distribution upgrade

This reverts commit df479e622f6791bc6a9f336eb28bec57722872d1.

Conflicts:
	ftfy/__init__.py