Qt: preferences: remove unused hidden member

(cherry picked from commit 89f039dd14b2bf8da73b08bc23ded28da9044867)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/dialogs/preferences.hpp

@@ -54,8 +54,6 @@ public:
 #endif
 
 private:
-    QGridLayout *main_layout;
-
     QWidget *main_panel;
     QHBoxLayout *main_panel_l;