Cast to DefaultButtonModel to access getGroup() is unnecessary

As of JDK-8182577, ButtonModel interface has the method getGroup(). Previously, whenever getGroup() was needed, the model object required casting to DefaultButtonModel.

There are a couple of places where model object is cast to DefaultButtonModel before calling getGroup() method.

BasicButtonUI.ButtonGroupInfo.getButtonGroupInfo() (lines 717-719):
            ButtonModel model = activeBtn.getModel();
            if (!(model instanceof DefaultButtonModel))
                return false;