Don't generate adjacent links in wp_head() for attachments.

props kovshenin.
fixes #21658.

git-svn-id: https://develop.svn.wordpress.org/trunk@28770 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/link-template.php

@@ -1655,7 +1655,7 @@ function adjacent_posts_rel_link( $title = '%title', $in_same_term = false, $exc
  *
  */
 function adjacent_posts_rel_link_wp_head() {
-	if ( ! is_single() ) {
+	if ( ! is_single() || is_attachment() ) {
 		return;
 	}
 	adjacent_posts_rel_link();