Disable KXMLGUI workaround for adding toolbars at ≥ KF 5.78

Since frameworks/kxmlgui!5 (merged), this workaround should no longer be necessary, so we can completely remove it in one or two years?

At least my KXMLGUI > 5.78 inserted the new annotation toolbar correctly when I removed it from part.rc and decreased part.rc to version="46".