ROADMAP: remove the protobuf line item
ROADMAP: remove the protobuf line item

As using protobuf has been decided against, this removes it from the
Roadmap items.

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

opencontainers/runtime-spec,hqhq/runtime-spec,cloudfoundry-incubator/specs,jonboulle/specs,vbatts/specs,ujuc/specs,hqhq/specs,laijs/specs,wking/opencontainer-specs,wking/opencontainer-runtime-spec,opencontainers/specs