Remove outdated AbstractDataPlugin::setDelegate(QtQuick stuff)

Summary:
* subclass DeclarativeDataPlugin replaced that need
* allows to drop dep of libmarblewidget to Qt5::Quick again

Follow-up for c9bf0b77

Reviewers: sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2714