In the latest Raspbian distributions the default syslog configuration writes cron messages, including success messages, in to /var/log/syslog. To eliminate this, edit /etc/rsyslog.conf
% sudo vi /etc/rsyslog.conf
uncomment the line for /var/log/cron.log, and add cron as an exclusion to /var/log/syslog as follows:
cron,* /var/log/cron *.*, cron,auth,authpriv.none /var/log/syslog
and then force a restart of syslog using
% sudo systemctl restart syslog
For housekeeping /var/log/cron.log also needs to be added to /etc/logrotate.conf, however this appears to be the default now in Raspbian.
Thanks to serverfault for this information.