efl_container: remove content_remove

efl_container: remove content_remove

Most of classes implements Efl.Container.content_remove are just calling "unpack"
except of "Efl.Canvas.Layout" and "Efl.Ui.Layout".
This patch remove the asymmetrical API and add content_remove API of
"Efl.Canvas.Layout" and "Efl.Ui.Layout" that child can be remove without efl_part
APIs.

ref T7576

Reviewed-by: Marcel Hollerbach <marcel-hollerbach@t-online.de>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Differential Revision: https://phab.enlightenment.org/D7913