MongoDB Cluster drop-down list doesn't get updated on removal.

After removing all servers from a cluster, the drop-down list will still include the cluster name.

Steps to reproduce:

Install pmm-client and use pmm-admin command to monitor a node, and add it to a cluster

pmm-admin add mongodb:metrics --uri "127.0.0.1:20369" --cluster mongoClusterPMM2 bm-support02 

Remove the exporter

pmm-admin remove mongodb:metrics bm-support02 

Go to the MongoDB Overview dashboard and see how it is still listed (even after some time has passed, and there are no more metrics.

Some screenshots attached to show this (in this environment I had also created another cluster with another node being monitored, this is why you will see listings for bm-support01 and mongoClusterPMM.

Suggested fix:

Don't include cluster names for instances that don't exist.