MEMBEROF: Implement delete operation for ghost users
The memberof plugin did only expand the ghost users attribute to
parents when adding a nested group, but didn't implement the reverse
This bug resulted in users being reported as group members even
after the direct parent went away as the expanded ghost attributes were
never removed from the parent entry.
When a ghost entry is removed from a group, all its parent groups are
expired from the cache by setting the expire timestamp to 1. Doing so
would force the SSSD to re-read the group next time it is requested in
order to make sure its members are really up-to-date.