Cleanup Qt branches on old unsupported version

Summary:
This cleans the remaining #ifdef on older Qt versions. We are
currently requiring 5.5.1 as a minimum.

Backport of core PR14725.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6569