fetcher: Consider removing `PrototypeFromLink`

[willscott]

PrototypeFromLink(link ipld.Link) (ipld.NodePrototype, error) is a weird thing to have as part of the fetcher interface. This seems to be a function of the active ipld linksystem, and it seems unclear why this is something that belongs on the fetcher public interface. We should document or consider where the right place for this to live would be.