rpc: deduplicate WriteHDKeypath() used in decodepsbt

Summary:
The functionality is already provided in the BIP32 utility library util/bip32.h
with the exact same name and function signature.

This is a backport of core#19555

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10015