drop credit_cards first_name and last_name fields

It was intended for this to happen eventually: https://github.com/spree/spree/pull/2807#issuecomment-16345435
It's been causing confusion for us and some others in a couple ways so it seems like a good time to drop them.
I assume you found an answer to your question here? https://github.com/spree/spree/issues/4728#issuecomment-45090991

It's not clear to me that we need to_active_merchant or that the comment above is accurate