Remove SetModal() native implementation.

widget/src/gtk/nsWidget.h

@@ -65,7 +65,6 @@ class nsWidget : public nsBaseWidget
     NS_IMETHOD Destroy(void);
     nsIWidget* GetParent(void);
 
-    NS_IMETHOD SetModal(void);
     NS_IMETHOD Show(PRBool state);
     NS_IMETHOD IsVisible(PRBool &aState);