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/models/inventory.py

@@ -29,7 +29,6 @@ from awx.main.constants import CLOUD_PROVIDERS
 from awx.main.consumers import emit_channel_notification
 from awx.main.fields import (
     ImplicitRoleField,
-    JSONBField,
     SmartFilterField,
     OrderedManyToManyField,
 )
@@ -488,7 +487,7 @@ class Host(CommonModelNameNotUnique, RelatedJobsMixin):
         editable=False,
         help_text=_('Inventory source(s) that created or modified this host.'),
     )
-    ansible_facts = JSONBField(
+    ansible_facts = models.JSONField(
         blank=True,
         default=dict,
         help_text=_('Arbitrary JSON structure of most recent ansible_facts, per-host.'),