Remove incorrect usages of link options destination

The destination field in link options is initialized to the empty string and can no longer be nullable. Therefore the null check is no longer valid, since the destination is always set to a non-null value. Secondly, we already initialize to the empty string and therefore don't need to reassign the value to the empty string during prefetch loading.

This is a follow-up to ad28192.

Closes #11726 (again).

Author: TimvdLippe

source

@@ -16681,9 +16681,6 @@ interface <dfn interface>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
    <li><p><span>Assert</span>: <var>options</var>'s <span data-x="link options href">href</span> is
    not the empty string.</p></li>
 
-   <li><p>If <var>options</var>'s <span data-x="link options destination">destination</span> is
-   null, then return null.</p></li>
-
    <li>
     <p>Let <var>url</var> be the result of <span>encoding-parsing a URL</span> given
     <var>options</var>'s <span data-x="link options href">href</span>, relative to
@@ -28722,9 +28719,6 @@ document.body.appendChild(wbr);</code></pre>
    <li><p>Let <var>options</var> be the result of <span data-x="create link options from
    element">creating link options</span> from <var>el</var>.</p></li>
 
-   <li><p>Set <var>options</var>'s <span data-x="link options destination">destination</span> to
-   the empty string.</p></li>
-
    <li><p>Let <var>request</var> be the result of <span data-x="create a link request">creating a
    link request</span> given <var>options</var>.</p></li>