TitleLDAP: Once synced contact groups can not be deleted anymore

When using the LDAP sync to manage users and contact group memberships one needs to create the contact groups to be synced from LDAP manually. From the first sync after creation the group memberships are synchronized by the LDAP sync. Since the membership attribute of all LDAP users is locked, it is not possible remove the assignment to the group. This makes it impossible to delete the group while there are LDAP users for this group. It should be possible to delete the group after confirming a warning message.