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 (this sounds like
  what you're suggesting here). 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]: #390
[2]: #390 (comment)

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

manifest-list.md

@@ -2,7 +2,6 @@
 
 The manifest list is a higher-level manifest which points to specific [image manifests](manifest.md) for one or more platforms.
 While the use of a manifest list is OPTIONAL for image providers, image consumers SHOULD be prepared to process them.
-A client will distinguish a manifest list from an image manifest based on the Content-Type returned in the HTTP response.
 
 This section defines the `application/vnd.oci.image.manifest.list.v1+json` [media type](media-types.md).