You can remove fortitray from System Tray icons by clicking on
Application Launcher --> search for "System Tray Settings - Plasma" --> go to Entries --> under Application Status you will find fortitray along with other applications.
Select "Always hidden" option to remove it from system tray.
If you want to completely disable Forticlient from autostarting, you have to disable forticlient service by using
sudo systemctl disable forticlient.service
Note: Please keep in mind that, if you want to run FortiClient again, you have to first start service with "sudo systemctl start forticlient.service" and then open FortiClient Console.