system: remove mention of extended account usage

It's only partially true and will be compeltely wrong in
the mid-term future.  ;)

(cherry picked from commit 6855a212)

src/www/system_usermanager.php

@@ -954,9 +954,6 @@ $( document ).ready(function() {
                                         "the webConfigurator can be assigned directly or inherited from group memberships. " .
                                         "An icon that appears grey indicates that it is a system defined object. " .
                                         "Some system object properties can be modified but they cannot be deleted."); ?>
-                          <br /><br />
-                          <?=gettext("Accounts created here are also used for other parts of the system " .
-                                        "such as OpenVPN, IPsec, and Captive Portal.");?>
                       </td>
                     </tr>
                   </tbody>