Removing rel='ostatus:attention' in favor of Salmon's rel='mentioned'

This was introduced in StatusNet 0.9.x, which is the oldest branch
on the web today as far as I know.

lib/activity.php

@@ -627,8 +627,6 @@ class Activity
             }
 
             foreach ($this->context->attention as $attnURI) {
-                $xs->element('link', array('rel' => ActivityContext::ATTENTION,
-                                           'href' => $attnURI));
                 $xs->element('link', array('rel' => ActivityContext::MENTIONED,
                                            'href' => $attnURI));
             }