Fully remove issue alert columns on NotificationMessage

[ceorourke]

Follow up to #115290 to fully remove the columns.

[wedamija]

lgtm, usual reminder to not merge until the removal pr deploys

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/notifications/migrations/0007_delete_issue_alert_columns_notificationmessage.py

for 0007_delete_issue_alert_columns_notificationmessage in notifications

--
-- Remove field rule_action_uuid from notificationmessage
--
ALTER TABLE "sentry_notificationmessage" DROP COLUMN "rule_action_uuid" CASCADE;
--
-- Remove field rule_fire_history from notificationmessage
--
ALTER TABLE "sentry_notificationmessage" DROP COLUMN "rule_fire_history_id" CASCADE;