test: refactor: remove hex_str_to_bytes helper

Summary:
Use the built-in class method bytes.fromhex() instead, which is available since Python 3.0.

This is a backport of core#22593

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11136