mgmt: mcumgr: grp: img_mgmt: Remove dummy header

The dummy header was an opt-in feature to allow testing without a
real image but since we support testing on Qemu in CI which
supports flash partitions, there isn't a need for this option. Not
being deprecated as Kconfig would only have been used as part of
CI, not user applications.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>