ldap: remove parallel requests from rfc2307bis
    
    As this branch of code is not well tested, we've decided to just nuke it
    and, in the future, spend more time on improving the performance for the
    general case instead of maintaining this old "hack".
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3494
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>