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/main/utils/filters.py

@@ -188,13 +188,11 @@ class SmartFilter(object):
         '''
 
         def _json_path_to_contains(self, k, v):
-            from awx.main.fields import JSONBField  # avoid a circular import
-
             if not k.startswith(SmartFilter.SEARCHABLE_RELATIONSHIP):
                 v = self.strip_quotes_traditional_logic(v)
                 return (k, v)
 
-            for match in JSONBField.get_lookups().keys():
+            for match in models.JSONField.get_lookups().keys():
                 match = '__{}'.format(match)
                 if k.endswith(match):
                     if match == '__exact':