ref(dashboards): Drop DashboardTombstone table

[gggritso]

Drop the sentry_dashboardtombstone table from PostgreSQL via
SafeDeleteModel(DELETE).

Do not merge this until #114265 has been deployed and verified in production.

PR Stack

1. ref(dashboards): Remove DashboardTombstone model #114265 — Remove model + MOVE_TO_PENDING migration
2. ref(dashboards): Drop DashboardTombstone table #114267 ← this PR — DELETE migration to drop the table (deploy after ref(dashboards): Remove DashboardTombstone model #114265)

Refs DAIN-1567

[linear-code]

DAIN-1567 Remove `DashboardTombstone` model

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1074_remove_dashboardtombstone.py src/sentry/migrations/1075_delete_dashboardtombstone.py

for 1074_remove_dashboardtombstone in sentry

--
-- Alter field organization on dashboardtombstone
--
SET CONSTRAINTS "sentry_dashboardtomb_organization_id_fa5fbf63_fk_sentry_or" IMMEDIATE; ALTER TABLE "sentry_dashboardtombstone" DROP CONSTRAINT "sentry_dashboardtomb_organization_id_fa5fbf63_fk_sentry_or";
--
-- Moved model DashboardTombstone to pending deletion state
--
-- (no-op)

for 1075_delete_dashboardtombstone in sentry

--
-- Delete model DashboardTombstone
--
DROP TABLE "sentry_dashboardtombstone" CASCADE;

[wedamija]

This mostly lgtm, but you'll need to fix this test failure to make sure the table is really deleted

ValueError: Cannot determine database for deleted model sentry.DashboardTombstone (table: sentry_dashboardtombstone). This table must be added to historical_silo_assignments in src/sentry/db/router.py (or getsentry/db/router.py for getsentry models) with the appropriate SiloMode before the deletion migration can run.

[wedamija]

Migration 1075 lgtm, just need to rebase and clean up

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1075_delete_dashboardtombstone.py

for 1075_delete_dashboardtombstone in sentry

--
-- Delete model DashboardTombstone
--
DROP TABLE "sentry_dashboardtombstone" CASCADE;

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 0137160. Configure here.}

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1076_delete_dashboardtombstone.py

for 1076_delete_dashboardtombstone in sentry

--
-- Delete model DashboardTombstone
--
DROP TABLE "sentry_dashboardtombstone" CASCADE;