unistalling software on ubuntu server 18.04
The reverse of
sudo mv mattermost /opt
is
sudo rm -rf /opt/mattermost
Mind though that a source install tends to have a document in the directory explaining how to uninstall (so check for a readme or uninstall document in /opt/mattermost/) as that should take precedence over the rm command.
Also make sure if there is a service you stop it first.
Remove the service with:
rm /lib/systemd/system/mattermost.service
systemctl daemon-reload