ipatests: remove invalid parameter from sssd.conf
    
    `use_fully_qualified_names` is not a valid parameter for `[sssd]` section
    of sssd.conf, it can be specified only in domain section.
    According to `man sssd.conf` it simply requires all requests to be fully
    qualified, otherwise no result will be found. It is irrelevant to the
    test scenario, so removing it.
    
    Related to: https://pagure.io/freeipa/issue/8219
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>