Revert "Better ID for notice activity"

This reverts commit 8cc4660bd9.

This seems like something Evan only did to make pump.io import notices easier,
or maybe he just wanted to get rid of the identi.ca URLs?

classes/Notice.php

@@ -1508,8 +1508,9 @@ class Notice extends Managed_DataObject
 
         if (Event::handle('StartNoticeAsActivity', array($this, &$act))) {
 
-            $act->id      = TagURI::mint("post:".$this->id);
+            $act->id      = $this->uri;
             $act->time    = strtotime($this->created);
+            $act->link    = $this->bestUrl();
             $act->content = common_xml_safe_str($this->rendered);
 
             $profile = $this->getProfile();