drop image attribute registries, rename "image attribute subpacket" to "JPEG attribute subpacket"
@letoams wrote a reasonable message advocating for making clearer text around handling some as-yet-unspecified header fields in the Image Attribute header. That prompted me to think about a simplifying cleanup which just acknowledged the current reality and made "Image Attributes" JPEGs and froze these fields in time. If some future image format (or other attribute type) is desirable, they can just be added to the table of attribute subpacket types directly.
This is closer to the "have one joint and use it regularly" approach that should make for a simpler wire format standard.
(i don't think this is in-scope for the current WG charter, and i've marked it as such)