Don't include HMAC in Refund paths

Don't include HMAC in Refund paths

Refunds are typically communicated via QR code, where a smaller size is
desirable. Make the HMAC in OutboundPayment data optional such that it
is elided from blinded paths used in refunds. This prevents abandoning
refunds if the reader sends an invoice_error instead of an invoice
message. However, this use case isn't necessary as the corresponding
outbound payment will either timeout when the refund expires or can be
explicitly abandoned by the creator.