[ticket/11355] Wrong error message when no user is selected.

In ACP group membership management, if you select 'remove member
from group' without selecting any users and submit,it will display
a wrong error after confirmbox. Since no users are selected, this
should not go even until confirmbox. Added a new condition to check
weather any users are selected when the action is 'deleteusers'
in acp_groups.php and disply a correct error.

PHPBB3-11355