https://pve.proxmox.com/pve-docs/pve-admin-guide.html#pvecm_separate_node_without_reinstall minus the last few steps.
systemctl stop pve-cluster corosync
pmxcfs -l
rm -rf /etc/corosync/*
rm /etc/pve/corosync.conf
killall pmxcfs
systemctl start pve-clusterShould do the trick.
