ipaplatform: Drop the base authconfig class
    
    As authconfig is a distro-specific tool there is no incentive for
    implying that other platforms should implement any authconfig
    implementation of their own.
    
    Part of: https://fedorahosted.org/freeipa/ticket/4052
    
    Reviewed-By: Petr Viktorin <pviktori@redhat.com>