client: remove unused libcurl build dependency
    
    The configure script checks for libcurl, but it is never actually used
    anywhere.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>