[backport] qt: Remove obsolete QModelIndex::child() #16707

Summary:
c53667764ea65c0f8661358759b0124f808ce587 qt: Remove obsolete QModelIndex::child() (Hennadii Stepanov)

Pull request description:

The QModelIndex::child() member function is obsolete since Qt 5.12.

This PR removes it, does not change behavior and keeps compatibility with Qt 5.5.1.

This is a backport of Core PR16707

Test Plan:

ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5385