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.
@huoxito

> I assume you found an answer to your question here? #4728 (comment)

I couldn't *find* any usage of to_active_merchant at least.  And I verified that for Braintree the ["creditcard"](https://github.com/Shopify/active_merchant/blob/v1.42.3/lib/active_merchant/billing/gateways/braintree_blue.rb#L113) object that ActiveMerchant receives is a Spree::CreditCard rather than an ActiveMerchant::Billing::CreditCard.  And that means that Spree::CreditCard needs to have "first_name" & "last_name" methods.

> It's not clear to me that we need to_active_merchant or that the comment above is accurate

I improved the comment a bit, but I think it's accurate and I think what we really need is for "to_active_merchant" to live inside spree_gateway and for spree_gateway to use it when sending credit cards to ActiveMerchant.  ActiveMerchant [seems to expect ActiveMerchant::Billing::CreditCard objects](https://github.com/Shopify/active_merchant/blob/v1.42.3/GettingStarted.md#handling-credit-cards).  Or at least [USA EPay](https://github.com/Shopify/active_merchant/blob/v1.42.3/lib/active_merchant/billing/gateways/usa_epay_advanced.rb#L1244) and [iTransact](https://github.com/Shopify/active_merchant/blob/v1.42.3/lib/active_merchant/billing/gateways/itransact.rb#L328) both have code that breaks otherwise.

How about I create a separate PR for dropping "to_active_merchant"?  It's hard to be absolutely certain that it's never used anywhere so it would be nice to make that a standalone change.