LibGUI: Remove confusing GModelNotification concept

This was a bad idea and it didn't stick. Instead we should just use the
simple "on_foo" hook functions like we do for everything else. :^)

Applications/SystemMonitor/ProcessTableView.h

@@ -17,7 +17,4 @@ public:
     void refresh();
 
     Function<void(pid_t)> on_process_selected;
-
-private:
-    virtual void model_notification(const GModelNotification&) override;
 };