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/metadata.py

@@ -6,6 +6,7 @@ from uuid import UUID
 
 # Django
 from django.core.exceptions import PermissionDenied
+from django.db.models import JSONField
 from django.db.models.fields import PositiveIntegerField, BooleanField
 from django.db.models.fields.related import ForeignKey
 from django.http import Http404
@@ -22,7 +23,7 @@ from rest_framework.request import clone_request
 
 # AWX
 from awx.api.fields import ChoiceNullField
-from awx.main.fields import JSONField, ImplicitRoleField
+from awx.main.fields import ImplicitRoleField
 from awx.main.models import NotificationTemplate
 from awx.main.utils.execution_environments import get_default_pod_spec