chore(savedsearch): Remove SavedSearch model

[ceorourke]

Follow up to #118883 to remove the SavedSearch model. A final follow up will do deletion_action=DeletionAction.DELETE to remove the table from Postgres.

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[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/1128_remove_savedsearch.py

for 1128_remove_savedsearch in sentry

--
-- Alter field organization on savedsearch
--
SET CONSTRAINTS "sentry_savedsearch_organization_id_3fd9781e_fk_sentry_or" IMMEDIATE; ALTER TABLE "sentry_savedsearch" DROP CONSTRAINT "sentry_savedsearch_organization_id_3fd9781e_fk_sentry_or";
--
-- Moved model SavedSearch to pending deletion state
--
-- (no-op)