c53667764ea65c0f8661358759b0124f808ce587 qt: Remove obsolete QModelIndex::child() (Hennadii Stepanov)
Pull request description:
The `QModelIndex::child()` member function is [obsolete](https://doc.qt.io/qt-5.12/qmodelindex-obsolete.html) since Qt 5.12.
This PR removes it, does not change behavior and keeps compatibility with [Qt 5.5.1](https://github.com/bitcoin/bitcoin/pull/15393).
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This is a backport of Core PR16707