Simplify CargoBayProductRequestDelegate init

Unless I’m missing something here, these blocks are `(copy)` properties, so I don’t see why we should redefine them. 

I understand we need to check if these blocks are nil before actually calling them, but this verification already exists where they are being used in `request:didFailWithError:` and `productsRequest:didReceiveResponse:`. So we’re checking twice, which seems unnecessarily redundant to me.