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. Web interfaces
allows to remove invoices, it led to removed balance => fixed.

lib/NGCP/Schema/Result/contract_balances.pm

@@ -75,7 +75,7 @@ __PACKAGE__->belongs_to(
   {
     is_deferrable => 1,
     join_type     => "LEFT",
-    on_delete     => "CASCADE",
+    on_delete     => "SET NULL",
     on_update     => "CASCADE",
   },
 );