manifest-list: Drop HTTP Content-Type sentence
manifest-list: Drop HTTP Content-Type sentence

As Adrian Colley points out [1], clients may not be aquiring the
manifest over HTTP.  In most cases, folks will know (from a
Content-Type header, a descriptor media type, or other means) what
type of blob they're dealing with before they look at the blob.  I
expect client behavior like [2]:

  If you can verify the digest, ignore Content-Type.  If you can't
  verify the digest, respect the Content-Type and require it to match
  your expected media type (if any).

But in the absence of an HTTP-API spec I don't think we need to talk
about this at all.

[1]: https://github.com/opencontainers/image-spec/issues/390
[2]: https://github.com/opencontainers/image-spec/issues/390#issuecomment-254029311

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,vbatts/oci-image-spec,opencontainers/image-spec,jonboulle/image-spec