UI/Qt: Remove unused/undefined declaration from Qt settings

Author: https://github.com/trflynn89
Commit: d9ebd44924
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6175
Reviewed-by: https://github.com/gmta ✅

UI/Qt/Settings.h

@@ -46,7 +46,6 @@ public:
 
 signals:
     void show_menubar_changed(bool show_menubar);
-    void enable_do_not_track_changed(bool enable);
 
 protected:
     Settings();