elm_map: Fix work of elm_map_overlay_icon_set if icon is NULL.

elm_map: Fix work of elm_map_overlay_icon_set if icon is NULL.

Summary:
Delete check on NULL for icon object due to incorrect work. Function must delete
icon in map overlay (according to documentation) if we set NULL, but with this check -
nothing happening and after elm_map_overlay_icon_get we have not NULL returned value.
@fix

Reviewers: cedric, Hermet, raster, jpeg

Reviewed By: cedric

Subscribers: artem.popov

Differential Revision: https://phab.enlightenment.org/D4720

Change-Id: I0079d88374e9eda920b6eef143744cdc902fe773