toolbar: item content unset function called when user content unset to toolbar item.
toolbar: item content unset function called when user content unset to toolbar item.

Summary:
It's related with https://phab.enlightenment.org/D2777

I missed doing on content_unset case.

Reviewers: Hermet

Reviewed By: Hermet

Differential Revision: https://phab.enlightenment.org/D2808

FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary