Scenario

On the cluster management page the option to delete the old collector is greyed out and the tool tip says it can't delete the last collector/node in the cluster.

Resolution

Use the link from the previous version's collector management UI URL.
 

<OPSVIEW_URL>/settings/#!/config/monitoringcollectors

 
Delete the collector and cluster from the link above then delete/remove the host from the Configuration > Hosts page.
Then edit your deploy files (opsview_deploy.yml, user_vars.yml, and others as appropriate) to either comment out or remove the lines for the deleted Collector.