International letters don’t have a choice of postage. Under the hood
they are either `europe` or `rest-of-world`.
So, for letters that we detect are international, this commit:
- removes the radios buttons that give users the choice of postage
- passes through either `europe` or `rest-of-world` to the API,
depending on what address we find in the letter
This will cause the API to 500 until it can accept `europe` or
`rest-of-world` as postage types, but this is probably OK because it’s
only our services that have international letters switched on at the
moment.