[apps] Factorize isDisplayingEmptyController

Because we can now, it might need to be de-factorized later, if more
classes inherit from alternate_empty_nested_menu_controller

apps/code/variable_box_controller.h

@@ -32,7 +32,6 @@ public:
 
   //AlternateEmptyNestedMenuController
   ViewController * emptyViewController() override { return &m_variableBoxEmptyController; }
-  bool isDisplayingEmptyController() override { return StackViewController::depth() == 2; } //TODO LEA factorize with math toolbox
 
   /* VariableBoxController */
   void loadFunctionsAndVariables(int scriptIndex, const char * textToAutocomplete, int textToAutocompleteLength);