I removed it->generation, sd->generation, it->walking, sd->walking

I removed it->generation, sd->generation, it->walking, sd->walking
flags.
This flags was used for deferring deletion of genlist items.
After deferred, items can be cleanup when _item_select() is called.
This flags was used for preventing crash when subitems are deleted right
after one item is deleted.

But this made genlist code too complicated and could hide internal
issues. One of the issues is incorrect use of EINA_INSTLIST_FOREACH (or
forloop). Because sub items can be deleted when one item is deleted, I
used whlie loop and EINA_INLIST_CONTAINER_GET(sd->items->last, xx) and
not defer the deletion. This has no problem evenif inlist sd->items is
changed inside the while-loop.