Remove DDD boilerplate

From @bricelam:

It is the professional opinion of the EF team that the DDD patterns employed in the Ordering.API and related projects are of no value (and possibly even result in negative value) and can safely be removed for the sake of maintainability.

This includes, but is not limited to:

- [ ] Removing the types under the [src/Ordering.Domain/SeedWork/](https://github.com/dotnet/eShop/tree/main/src/Ordering.Domain/SeedWork) directory
- [ ] Use actual CLR enums for [CardType](https://github.com/dotnet/eShop/blob/main/src/Ordering.Domain/AggregatesModel/BuyerAggregate/CardType.cs) and [OrderStatus](https://github.com/dotnet/eShop/blob/main/src/Ordering.Domain/AggregatesModel/OrderAggregate/OrderStatus.cs)
- [ ] Update [OrderQueries](https://github.com/dotnet/eShop/blob/main/src/Ordering.API/Application/Queries/OrderQueries.cs) to use EF LINQ queries
- Make ViewModel classes more idiomatic
- [ ] Consider Using `IdentityGuid` as the primary key (instead of the current surrogate key) for [Buyer](https://github.com/dotnet/eShop/blob/main/src/Ordering.Domain/AggregatesModel/BuyerAggregate/Buyer.cs)
- [ ] Review the unusual use of transactions and the SaveChanges call in [TransactionBehavior](https://github.com/dotnet/eShop/blob/main/src/Ordering.API/Application/Behaviors/TransactionBehavior.cs)