[ticket/17107] Fix PHP warning on legend output

This is a side issue revealed with the newly added viewonline functional test.
In current master branch group legend is being dumped to template with the
group helper->display_legend() method so plain LEGEND output is a leftover.

PHPBB3-17107

phpBB/viewonline.php

@@ -463,7 +463,6 @@
 $template->assign_vars(array(
 	'TOTAL_REGISTERED_USERS_ONLINE'	=> $user->lang('REG_USERS_ONLINE', (int) $logged_visible_online, $user->lang('HIDDEN_USERS_ONLINE', (int) $logged_hidden_online)),
 	'TOTAL_GUEST_USERS_ONLINE'		=> $user->lang('GUEST_USERS_ONLINE', (int) $guest_counter),
-	'LEGEND'						=> $legend,
 
 	'U_SORT_USERNAME'		=> append_sid("{$phpbb_root_path}viewonline.$phpEx", 'sk=a&sd=' . (($sort_key == 'a' && $sort_dir == 'a') ? 'd' : 'a') . '&sg=' . ((int) $show_guests)),
 	'U_SORT_UPDATED'		=> append_sid("{$phpbb_root_path}viewonline.$phpEx", 'sk=b&sd=' . (($sort_key == 'b' && $sort_dir == 'a') ? 'd' : 'a') . '&sg=' . ((int) $show_guests)),