zephyr: Remove scratch from flash_area_id_from_multi_image_slot

The mapping does not seem to be needed as scratch is opened directly,
and there is only one for all images anyway.
Simulator seems to error out when asked to identify scratch with
this function.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 06747985)