Drop ParseHashUV in favor of calling ParseHashStr

Summary:
The one existing call already validates get_str will
pass via checkObject.

This is a backport of Core PR13422

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4215