sqldb: delete wrong index names and add missing one

sqldb/sqlc/migrations/000001_invoices.down.sql

@@ -1,7 +1,3 @@
-DROP INDEX IF EXISTS invoice_payments_invoice_id_idx;
-DROP INDEX IF EXISTS invoice_payments_settled_at_idx;
-DROP TABLE IF EXISTS invoice_payments;
-
 DROP INDEX IF EXISTS invoice_htlc_custom_records_htlc_id_idx;
 DROP TABLE IF EXISTS invoice_htlc_custom_records;
 
@@ -11,6 +7,7 @@ DROP TABLE IF EXISTS invoice_htlcs;
 DROP INDEX IF EXISTS invoice_feature_invoice_id_idx;
 DROP TABLE IF EXISTS invoice_features;
 
+DROP INDEX IF EXISTS invoices_settled_at_idx;
 DROP INDEX IF EXISTS invoices_created_at_idx;
 DROP INDEX IF EXISTS invoices_state_idx;
 DROP INDEX IF EXISTS invoices_payment_addr_idx;