Unnecessary detach in QImage::setAlphaChannel(const QImage &alphaChannel)

While fixing QTBUG-4887, I noticed what appears to be an unnecessary call to detach() in QImage::setAlphaChannel. It happens just before an assignment to *this with only a call to convertToFormat in between. I think the call to convertToFormat has no effect on this, and the assignment to *this will itself deref any shared copy, so the detach call could just create an unnecessary copy of the existing image, which would take quite a long time.