Docs: Fix incorrect DocBlock for wp_get_attachment_url().

The documented `$pagenow` global isn't used in the function.

Props sebastian.pisula.
Fixes #35413.

git-svn-id: https://develop.svn.wordpress.org/trunk@36268 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -4858,8 +4858,6 @@ function wp_update_attachment_metadata( $post_id, $data ) {
  *
  * @since 2.1.0
  *
- * @global string $pagenow
- *
  * @param int $post_id Optional. Attachment ID. Default 0.
  * @return string|false Attachment URL, otherwise false.
  */