elm_gengrid: fix correct parent-child relation of item content

elm_gengrid: fix correct parent-child relation of item content

Summary:
see 1b2401849a11c5826dead44b4839ca2acb1dbec1 that said
`evas_object_smart_member_del` doesn't make sense here.
I guess `_sub_object_del` should have been used when item_all_contents_unset is
implemented.

Item content should have widget(gengrid) as the parent. but that relationship
is removed by c075b7caaac0c6cf8c1043312041afaf5.
See also, `_item_content_realize` of genlist.

Test Plan: make check

Reviewers: Hermet, SanghyeonLee, bu5hm4n

Reviewed By: Hermet

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8238