ref(dashboards): Drop DashboardTombstone table

[gggritso]

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

Do not merge this until #114266 has been deployed and verified in production.

PR Stack

1. ref(dashboards): Remove DashboardTombstone model #114265 — Remove model class and code references
2. ref(dashboards): Add MOVE_TO_PENDING migration for DashboardTombstone #114266 — MOVE_TO_PENDING migration
3. ref(dashboards): Drop DashboardTombstone table #114267 ← this PR — DELETE migration (deploy after ref(dashboards): Add MOVE_TO_PENDING migration for DashboardTombstone #114266)

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;