Stop using ABNF for the Origin header

As we do not have ABNF-compatible definitions for scheme, host, and port.

Author: annevk

fetch.bs

@@ -2819,14 +2819,8 @@ request <a for=/>header</a> indicates where a
 <a lt=fetch for=/>fetches</a>.
 <!-- Ian Hickson told me Adam Barth researched that -->
 
-<p>Its <a for=header>value</a> <a>ABNF</a>:
-
-<pre><code class=lang-abnf>
-Origin                           = origin-or-null
-
-origin-or-null                   = origin / %s"null" ; case-sensitive
-origin                           = <a for=url>scheme</a> "://" <a for=url>host</a> [ ":" <a for=url>port</a> ]
-</code></pre>
+<p>Its possible <a for=header>values</a> are all the return values of
+<a>byte-serializing a request origin</a>, given a <a for=/>request</a>.
 
 <p class=note>This supplants the definition in <cite>The Web Origin Concept</cite>. [[ORIGIN]]