http: relax writeEarlyHints validations
Removes the requirement that every call to writeEarlyHints include a link header. While the link header is clearly the most common usage of 103 Early Hints, I could find no requirement to include a link header as part of RFC8297.
Additionally this removes the existing incorrect validation of the Link header format in favor of only validating that it is a valid header value. While the validation could be updated to better match RFC8288 Section 3, it appears it would be the only place in the node.js code base where we proactively validate header values beyond verifying they are valid at the HTTP protocol layer.