memberof: keep memberOf attribute for nested member
If we have a member that is both direct and nested member,
memberOf attribute was removed if the direct membership
was deleted.
1)
user ----------> groupB -> groupC
-> groupA /
2)
user -> groupA -> groupB -> groupC
If we remove user->groupB from 1), we get 2) but groupB was still
removed from user memberOf attribute.
Resolves:
https://pagure.io/SSSD/sssd/issue/3636
Reviewed-by: Sumit Bose <sbose@redhat.com>