ivi-shell: remove content_observer leftover

content_observer_notification is removed by the commit:
193e301c74.

Therefore, this callback function is unused.

Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

ivi-shell/ivi-layout-export.h

@@ -168,11 +168,6 @@ typedef void (*surface_configure_notification_func)(
 			struct ivi_layout_surface *ivisurf,
 			void *userdata);
 
-typedef void (*ivi_controller_surface_content_callback)(
-			struct ivi_layout_surface *ivisurf,
-			int32_t content,
-			void *userdata);
-
 struct ivi_layout_interface {
 
 	/**