Hide `inputAccessoryToolbar` when there is no postal code for the current country
Hide `inputAccessoryToolbar` when there is no postal code for the current country

I observed this bug while testing. The toolbar with 'Next' (above the number
pad) was visible for countries without postal codes. When the paymentCell was completely
filled, the 'Next' button enabled, but did nothing when tapped.

Instead of special-casing on the `configuration.requiredBillingAddressFields`, just
use the logic already built into `STPAddressViewModel` to know whether there'll be
a field to change to.

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