Clear dependent form input when a payment_method is selected at checkout

Another bug diagnosed by #2852207: Billing same as shipping.

When a payment_method radio is clicked, form api considers the rebuilt form to be submitted, ignoring the default values, and causing empty values to be shown instead. For example, the checkout form is loaded, "Cash on delivery" is selected, as is "Billing same as shipping" (since its #default_value is TRUE). Then we click "Credit card", and "Billing same as shipping" is unchecked, because there is $user_input.

The solution is the same that Address module uses, after #ajax clear dependent user input.