registerProtocolHandler(): drop scheme from note

If you put a % in a scheme, it effectively turns the input from an absolute URL into a relative URL.

See web-platform-tests/wpt#24886 for context, again.

source

@@ -94282,7 +94282,7 @@ interface <dfn>Navigator</dfn> {
     <code>DOMException</code>.</p>
 
     <p class="note">This is forcibly the case if the <code data-x="">%s</code> placeholder is in the
-    scheme, host, or port components of the URL.</p>
+    host or port of the URL.</p>
    </li>
 
    <li><p>If the <span>resulting URL record</span>'s <span data-x="concept-url-scheme">scheme</span>