While Review Board 4.0 was early in development, we hit a case with

Django's MySQL backend where setting a BinaryField to binary contents

would trigger a warning. This was fixed in Django 1.10.5, but applied to

Django 1.6 as well.
We worked around this in a custom backend defined in Djblets that we

patched in in Review Board's settings. As we're no longer using any

versions of Django that are susceptible to this bug, we no longer need

to patch in the backend.
This change removes that, allowing the original backend to be used.

Tested an install running on MySQL.
Unit tests pass.