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