Unfortunately, after the addition of `supported_arches` item to some
of the image type definitions in `format-request-map.json`, it started
to leak to the `compose-request` object in the generated image test
case.
Ensure that the item does not leak to the generated image test case,
since it is relevant only for the generator script.
Signed-off-by: Tomas Hozza <thozza@redhat.com>