paymentsdb: fix duplicate interface check and down migration drop order

- Remove duplicate compile-time interface assertion for SQLStore.

- Fix the down migration to drop payment_intents before payments to
respect the foreign key dependency order. This was not a bug in the
first place bc we have the CASCADE when deleting payments.

sqldb/sqlc/migrations/000010_payments.down.sql

@@ -39,16 +39,16 @@ DROP INDEX IF EXISTS idx_htlc_payment_hash;
 DROP INDEX IF EXISTS idx_htlc_attempt_time;
 DROP TABLE IF EXISTS payment_htlc_attempts;
 
--- ─────────────────────────────────────────────
--- Drop payments table and its indexes.
--- ─────────────────────────────────────────────
-
-DROP INDEX IF EXISTS idx_payments_created_at;
-DROP TABLE IF EXISTS payments;
-
 -- ─────────────────────────────────────────────
 -- Drop payment intents table and its indexes.
 -- ─────────────────────────────────────────────
 
 DROP INDEX IF EXISTS idx_payment_intents_type;
-DROP TABLE IF EXISTS payment_intents;
+DROP TABLE IF EXISTS payment_intents;
+
+-- ─────────────────────────────────────────────
+-- Drop payments table and its indexes.
+-- ─────────────────────────────────────────────
+
+DROP INDEX IF EXISTS idx_payments_created_at;
+DROP TABLE IF EXISTS payments;