Revert "zephyr: Use flash_area_flatten in bs_custom_storage_erase"

Need to revert changes provided to support devices without erase
until problme with TF-M, missing implementation of flash_area_get_sector,
is resolved.

This reverts commit ecd48775.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>