elementary: fix unintentional visibility of elm_hover in combobox constructor

elementary: fix unintentional visibility of elm_hover in combobox constructor

Summary:
since fd9cf1d3b628f70d66ec85a4f2433c675d9ad243, All Eo canvas objects remain
visible by default.
but elm_hover doesn't want to be shown in combobox constructor.
showing elm_hover in constructor occurs unexpected behavior.
this fixes that bug.

Test Plan:
1. elementary_test - Combobox
2. Click the combobox to Expand the list.

Reviewers: jpeg, cedric

Reviewed By: cedric

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D5267

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>