Makefile: remove oci-validate-examples target
Makefile: remove oci-validate-examples target

This binary was removed in a previous commit but the make targets were
not. The make targets have now been removed and the `validate-examples`
targets run the schema tests on the spec files.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

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