HOW TO REMOVE PROXMOX CLUSTER

systemctl stop pve-cluster corosync
pmxcfs -l
rm -r /etc/corosync/*
rm /etc/pve/corosync.conf
killall pmxcfs
systemctl start pve-cluster

PROXMOX QUORUM, EVICT NODE
If quorum is lost you´ll see messages like "proxmox unable to open file '/etc/pve/nodes/"
If not gain temporary quorum you can reduce the number of live nodes for quorum

pvecm expected 1

Delete one node

pvecm delnode old-node-name

Show cluster status

pvecm status