Merge #13962: Remove unused dummy_tx variable from FillPSBT

Summary:
16bcc1b8237698c96b8ced2fa7eb76388c7ba85e Remove unused dummy_tx variable from FillPSBT (Carl Dong)

Pull request description:

Tree-SHA512: e7652126bca2e87d445d4190aa0b4192d9575bc0c280d063302ca420be51e7a04fcbc24e0e7f5ec1f18938f0a596901e1285e9afc9b33ca3da78177938791163

Backport of Core PR13962

https://github.com/bitcoin/bitcoin/pull/13962/files

Test Plan: ninja check

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D5156