Remove `instruction` from `PaymentInstruction`

[ethanwlee]

Considering how difficult it is to present what payment information is needed just by using the plaintext instruction in PaymentInstruction, we should commit to using links whenever OrderInstructions is a non-empty object. Doing so would mean that PFIs are responsible for creating payment widgets that can launched as web views by client applications. This allows PFIs to have full control over exactly how they collect payment details and prevents client applications from having to support a vague, general order instruction. This approach mirrors the known-customer-credential spec regarding web view use.

[mistermoe]

thanks for documenting this @ethan-tbd ! will land a PR with proposed spec text tomorrow 🤞🏽