serialization: remove combined spec
serialization: remove combined spec

The combined spec isn't really part of the image format and introduces
new objects that we don't have a schema for. Largely I think that it
should be replaced by something like the image layout.

https://github.com/opencontainers/image-spec/pull/94
Signed-off-by: Brandon Philips <74d2b51265fa19d338eb593a7555ec2b932e59c0@coreos.com>

opencontainers/image-spec,opencontainers/image-spec,jbouzane/image-spec,xiekeyang/image-tools,runcom/image-spec,jonboulle/image-spec,mattspencer-arm/image-spec,opencontainers/image-tools,opencontainers/image-tools,mattspencer-arm/image-spec,xiekeyang/image-tools,vbatts/oci-image-spec,vbatts/oci-image-spec,runcom/image-spec,jonboulle/image-spec,jbouzane/image-spec