KConfigWidgets: port away from KF5 deprecated API

Summary:
KAboutData::setProgramIconName is deprecated and says
"Use QApplication::windowIcon", so I removed the code
that reads from programIconName()

Test Plan: Builds

Reviewers: kossebau, elvisangelaccio, vkrause

Reviewed By: kossebau

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24965