MDL-45242 Lib: Replace calls to deprecated functions

In all cases changes have been kept to a minimum while not making
the code completely horrible. For example, there are many instances
where it would probably be better to rewrite a query entirely, but
I have not done that (in order to reduce the risk of changes).

blocks/online_users/classes/fetcher.php

@@ -86,7 +86,8 @@ class fetcher {
         }
         $params = array();
 
-        $userfields = \user_picture::fields('u', array('username', 'deleted'));
+        $userfieldsapi = \core\user_fields::for_userpic()->including('username', 'deleted');
+        $userfields = $userfieldsapi->get_sql('u', false, '', '', false)->selects;
 
         // Add this to the SQL to show only group users.
         if ($currentgroup !== null) {