Drop forcing the QtQuick scene backend to software rendering

The enum-argument taking method is gone in Qt6, the replacement might be
the QString argument overload or even QQuickWindow::setGraphicsApi().

Though with the original motivation now gone (the welcomepage plugin
implementation has switched to use QtWidgets) and other QtQuick-using
features (like KNewStuff dialog) seemingly not needing other QtWidgets-
based applications to force the "software" backend, keeping doing it
in KDevelop so far has no known reason.