It is no good idea to delete it. It would be better to rotate it. When you simply delete it, the open file handle stays in RAM and new log entries won't be written, because the file that the handle is using is missing. When you rotate it, your current file will be stored (and as best practice compressed) and a new one will be initialized with a new file handle so that logging keeps working.The message file is very huge now - is it save to delete it?
Do you see a /etc/logrotate.d/syslog file? Then you can probably run
logrotate -f /etc/logrotate.d/syslog
to rotate the file.