list: added comments about an unused function.

list: added comments about an unused function.

Fixing this issue is not trivial and will bring another issues like crashing.
So it is better to fix this in a development phase by refactoring list.

To fix this issue, the following are needed:
1. it->walking concept should be adopted instead of using just sd->walking.
   sd->walking was introduced in beb418d6
2. elm_widget_item_del() should be called instead of the combination of
   _elm_list_item_free() + elm_widget_item_free()
   This was introduced in f343011d