Remove order.total

Up to now, we don't have a best-practise way to calculate this on the
fly.

orders/db/schema.cds

@@ -5,7 +5,6 @@ namespace sap.capire.bookshop;
 entity Orders : cuid, managed {
   OrderNo  : String @title:'Order Number'; //> readable key
   Items    : Composition of many OrderItems on Items.parent = $self;
-  total    : Decimal(9,2) @readonly;
   currency : Currency;
 }