ColumnView: Restore runtime dispatch of removeItem

QML runtime dispatching of overloaded functions wasn't implemented for
QObject types until Qt 6.5.3, so we can't rely on it yet. Revert to the
old behavior of manual dispatch via QVariant conversions.

See also: https://codereview.qt-project.org/c/qt/qtdeclarative/+/481934

Partially reverts 9a9d9a20