The following command should delete any file in /tmp, name of which is starting with "sess_", created or modified longer than 7 days ago. Any session state file persisting longer than 7 days, is quite unusual in my opinion and deletion of it should not cause any problems. But you should be the judge of how many days worth of those files you want to keep, by changing 7 to some other number in the command:
find /tmp -name "sess_*" -mtime +7 | xargs rm