`Cart.removeItem(…)` should not return any value

[odrotbohm]

It's more logical to return an empty CartItem to capture the context of the operation but indicate that a zero quantity is the result of the operation. Currently, we return the CartItem in its original state.

[odrotbohm]

CartItem does not allow zero quantities anymore, which rules it out as a return value. In that light, I think the current arrangement is fine. The only inconsistency now is that all our methods returning an Optional<CartItem> only return a non-empty Optional if the CartItem contained in it, is still attached to the Cart. That's not the case for a removal. I'll take this ticket to remove the return value of Cart.remove(…) entirely to avoid any ambiguities.