Ticket 49099 - fix configure.ac due to NS change
    
    Bug Description:  During the ns change, apparently make clean
    and autoreconf were not enough to check this was fine. I missed
    some removal of options.
    
    Fix Description:  Remove the enable_nunc_stans line, and libevent.m4
    
    https://pagure.io/389-ds-base/issue/49099
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)