This partially reverts https://github.com/moby/moby/pull/37350
Although specs.Platform is desirable in the API, there is more work
to be done on helper functions, namely containerd's platforms.Parse
that assumes the default platform of the Go runtime.
That prevents a client to use the recommended Parse function to
retrieve a specs.Platform object.
With this change, no parsing is expected from the client.
Signed-off-by: Tibor Vass <tibor@docker.com>
facad55744