Consistently use the QT_VERSION_CHECK macro

Summary:
We no longer need to remember how the Qt version number is built. It is
also easier to read.

Depends on D6569.

Test Plan:

ninja all check

Grepped for QT_VERSION to ensure there is no remaining case.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6570