MT#8433 Don't clear contract_balances on invoice deletion.

As far as I remember it is rudiment of old invoicing schema,
when balance supposed to be changed on invoice issue. So now
we allow to remove invoices from web interface and clear
accountent balance => fixed.

db_scripts/diff/15092.down

@@ -0,0 +1,3 @@
+USE billing;
+alter table contract_balances drop foreign key `cb_invoiceid_ref`;
+alter table contract_balances add CONSTRAINT cb_invoiceid_ref FOREIGN KEY (`invoice_id`) REFERENCES `invoices` (`id`) on delete CASCADE ON UPDATE CASCADE;

db_scripts/diff/15092.up

@@ -0,0 +1,3 @@
+USE billing;
+alter table contract_balances drop foreign key `cb_invoiceid_ref`;
+alter table contract_balances add CONSTRAINT cb_invoiceid_ref FOREIGN KEY (`invoice_id`) REFERENCES `invoices` (`id`) on delete set null ON UPDATE CASCADE;