chore(alerts): Remove metric alert constraint on NotificationMessage

[ceorourke]

Now that we've cleaned up usage we are no longer using the metric alert columns on this model and are exclusively writing to action and group for workflow engine notifications. Remove the constraints for metric alert notification messages so that we can remove the incident and trigger_action columns in follow ups. This is slightly risky in that technically without the constraint someone could write incorrect fields, but I'll be doing these migrations one after the other so the risk is minimal. I am planning to make action and group both not nullable.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/notifications/migrations/0008_remove_metric_alert_constraints_notificationmessage.py

for 0008_remove_metric_alert_constraints_notificationmessage in notifications

--
-- Remove constraint singular_parent_message_per_incident_and_trigger_action from model notificationmessage
--
DROP INDEX CONCURRENTLY IF EXISTS "singular_parent_message_per_incident_and_trigger_action";
--
-- Remove constraint notifmsg_metric_or_workflow_exclusive from model notificationmessage
--
ALTER TABLE "sentry_notificationmessage" DROP CONSTRAINT "notifmsg_metric_or_workflow_exclusive";

[wedamija]

Do we still need some of these constraints? Or are they all unused now?

[ceorourke]

I'll be removing incident and trigger action so the only constraint we might want is that both action and group are set but I intend to enforce that by removing the nullability on them.