Remove direct bitcoin calls from qt/intro.cpp

Summary:
Partial backport of Core PR 10244
https://github.com/bitcoin/bitcoin/pull/10244/commits/d7c2c9594897c39df6739b92610dfb5a7a1cb3ec
Progress towards T417
Depends on D2818

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

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2819