The postgres database is still there, you need to remove it first to reset mattermost:
Stop mattermost:
systemctl stop mattermost
Enter postgres94 psql:
su - postgres -c "scl enable rh-postgresql94 -- psql --port=55432"
Drop database:
drop database mattermost;
Exit psql:
\q
Reconfigure Mattermost:
signal-event nethserver-mattermost-update