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/api/serializers.py

@@ -97,7 +97,7 @@ from awx.main.models import (
 )
 from awx.main.models.base import VERBOSITY_CHOICES, NEW_JOB_TYPE_CHOICES
 from awx.main.models.rbac import get_roles_on_resource, role_summary_fields_generator
-from awx.main.fields import ImplicitRoleField, JSONBField
+from awx.main.fields import ImplicitRoleField
 from awx.main.utils import (
     get_type_for_model,
     get_model_for_type,
@@ -1718,7 +1718,7 @@ class InventorySerializer(LabelsListMixin, BaseSerializerWithVariables):
     def validate_host_filter(self, host_filter):
         if host_filter:
             try:
-                for match in JSONBField.get_lookups().keys():
+                for match in models.JSONField.get_lookups().keys():
                     if match == 'exact':
                         # __exact is allowed
                         continue