ivi-shell: remove a code which expects only a screen in the system.

It breaks from wl_list_for_each of screens when the frist screen found.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>