Fix 'Attribute history events accumulated' warning.

Summary:
Sometimes we see this warning:

/usr/local/lib/python2.7/dist-packages/sqlalchemy/orm/session.py:1824:
SAWarning: Attribute history events accumulated on 1 previously clean instances
within inner-flush event handlers have been reset, and will not result in
database updates. Consider using set_committed_value() within inner-flush event
handlers to avoid this warning.

I think this is because the after_flush listener in versioned_session calls
Transaction.set_extra_attrs, which in turn updates the Transaction object's
namespace attribute. Because of the now-removed backref, this would induce an
update to the corresponding Namespace object's attribute. That's the 'attribute
history event' that the warning is talking about. (You can instrument the
after_flush listener and see that after creating revisions, session.dirty
contains a Namespace instance that it did not before.)

Anyways, we don't seem to be using the backref, so remove it.

Test Plan:
Previously the warning would trigger on contacts sync. Now it does
not. Unit tests passing.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D66