Replace all usage of customized json fields with the Django builtin

The event_data field on event models, however, is getting an
overridden version that retains the underlying text data type for the
column, to avoid a heavy data migration on those tables.

Also, certain of the larger tables are getting these fields with the
NOT NULL constraint turned off, to avoid a long migration.

Remove the django.utils.six monkey patch we did at the beginning of
the upgrade.

awx/__init__.py

@@ -6,8 +6,6 @@ import os
 import sys
 import warnings
 
-import six
-
 from pkg_resources import get_distribution
 
 __version__ = get_distribution('awx').version
@@ -37,9 +35,6 @@ else:
     from django.db.models import indexes
     from django.db.backends.utils import names_digest
     from django.db import connection
-    from django import utils
-
-    utils.six = six  # FIXME: monkey patch to get us through for now
 
 if HAS_DJANGO is True: