Fix delete group cleans up role assignments with LDAP

When the LDAP assignment backend was used and a group was deleted,
the role assignments for the group were not getting cleaned up. The
role assignments for the group should have been removed along with the
group.

Change-Id: Ie506659c2b21e6a816335b00642fa4d0921a184d
Closes-Bug: #1366211
Merge "Fix delete group cleans up role assignments with LDAP"