SUDO: Drop logic to disable the backend in case the provider is not set
    
    As sudo responder requires some period task in the backend, has been
    decided (a long time ago) to disable it in the backend unless it's
    explicitly configured.
    
    Following Pavel Březina's suggestion, as it's one of our main features,
    let's treat it exactly as we treat any other responder and in case the
    admins want to disable it they should explicitly use:
    "sudo_provider = None".
    
    Related:
    https://fedorahosted.org/sssd/ticket/2243
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>