Remove direct bitcoin access from qt/guiutil.cpp

Summary:
Partial backport of Core PR 10244
https://github.com/bitcoin/bitcoin/pull/10244/commits/e872c93ee87477130fb877da1a536b4c693bbab9
Progress towards T417
Depends on D2887

Test Plan:
make check
ninja check
bitcoin-qt # sanity checks

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2888