Hi you can use this (I have used it last week on PVE 5.0 beta1):

### Removing cluster configuration:

(Perform the following steps on each node)

## stop the services

systemctl stop pvestatd.service
systemctl stop pvedaemon.service
systemctl stop pve-cluster.service
systemctl stop corosync
systemctl stop pve-cluster

## edit through sqlite, check, delete, verify

$ sqlite3 /var/lib/pve-cluster/config.db
sqlite> select * from tree where name = 'corosync.conf';
254327|0|254329|0|1480944811|8|corosync.conf|totem {
version: 2
[...]
sqlite> delete from tree where name = 'corosync.conf';
sqlite> select * from tree where name = 'corosync.conf';
sqlite> .quit

## Remove directories

pmxcfs -l
rm /etc/pve/corosync.conf
rm /etc/corosync/*
rm /var/lib/corosync/*
rm -rf /etc/pve/nodes/*

Dont forget to remove nodes that you dont want from /etc/pve/priv/authorized_keys

Reboot the servers and start again.