private/public we must remove irrelevant permissions (a public album
can have hidden permission, ie list of groups/users permitted). I've
also found a bug in the algorithm that selects users/groups to delete
(reverse parameters in array_diff).
git-svn-id: http://piwigo.org/svn/branches/2.5@24986 68402e56-0260-453c-a942-63ccdbb3a9ee