CRON[21587]: pam_unix(cron:session): session opened for user root by (uid=0)
CRON[21587]: pam_unix(cron:session): session closed for user root
CRON[21587]: pam_unix(cron:session): session closed for user root
This type of logs makes you hard to read the log by you can whe you are looking for information logged by ssh.
To strip off this logs you can simply edit /etc/pam.d/cron (using sudo since you need to root privileges) and comment out the line:
@include common-session