check for single-label domains only during server install
    
    The fix for https://pagure.io/freeipa/issue/7207 and
    https://pagure.io/freeipa/issue/7598 added checks against single-label
    domains in client, server and replica installs. This prevents client
    enrollment to existing topologies with single-label domain.
    
    This commit removes those fixes on ipa-4-6 branch. Server installation
    with single-label domain will still be refused, but client enrollment
    will succeed.
    
    Fixes: https://pagure.io/freeipa/issue/8058
    Reviewed-By: Francois Cami <fcami@redhat.com>