Remove remaining Heartbeat API experimental notices

Props DH-Shredder.
Fixes #28626.

Built from https://develop.svn.wordpress.org/trunk@28872


git-svn-id: http://core.svn.wordpress.org/trunk@28672 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/ajax-actions.php

@@ -11,7 +11,7 @@
 //
 
 /**
- * Ajax handler for the (experimental) Heartbeat API in
+ * Ajax handler for the Heartbeat API in
  * the no-privilege context.
  *
  * Runs when the user is not logged in.
@@ -2353,7 +2353,7 @@ function wp_ajax_send_link_to_editor() {
 }
 
 /**
- * Ajax handler for the (experimental) Heartbeat API.
+ * Ajax handler for the Heartbeat API.
  *
  * Runs when the user is logged in.
  *