Comment out adding the Mahara version to mail headers sent out.

There's no need to reveal this information, as the Mahara version is not revealed elsewhere.
(cherry picked from commit fb830d75)

htdocs/lib/user.php

@@ -270,7 +270,8 @@ function email_user($userto, $userfrom, $subject, $messagetext, $messagehtml='')
 
     $mail = new phpmailer();
 
-    $mail->Version = 'Mahara ' . get_config('release'); 
+    // Leaving this commented out - there's no reason for people to know this
+    //$mail->Version = 'Mahara ' . get_config('release');
     $mail->PluginDir = get_config('libroot')  . 'phpmailer/';
     
     $mail->CharSet = 'UTF-8';