Removing unused code from access-control-allow-lists.php in XHR LayoutTests

Bug: 
Change-Id: I026fc3a546afe4699452ced6810b187addeb576b
Reviewed-on: https://chromium-review.googlesource.com/578537Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Austin James Ahlstrom <aahlstrom@google.com>
Cr-Commit-Position: refs/heads/master@{#488180}

third_party/WebKit/LayoutTests/http/tests/xmlhttprequest/resources/access-control-allow-lists.php

@@ -26,6 +26,4 @@ foreach ($_SERVER as $name => $value)
     }
 }
 
-$headers['get_value'] = isset($_GET['get_value']) ? $_GET['get_value'] : '';
-
 echo json_encode( $headers );