remove use of getDefaultChangeType in paymentserver.cpp

Summary:
As requested in review of D9974:

make the changes to BIP70 related code in its own diff

This removes the last remaining occurence of getDefaultChangeType to enable backporting core#19396.
Our codebase only uses OutputType::LEGACY, so this change is trivial.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9976