boot: Remove pointless slot identification

In the boot_image_validate_encrypted there was call to
flash_area_id_to_multi_image_slot, which tries to figure out
slot index from flash area and image index, and the result of the
call was not used for anything as slot index is hardcoded in the
next call to be 1 (secondary).

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 4da4a72c)