Fix a bug for etcd removal machine controller [Etcd Proxy]

[nasusoba]

Bug found
The cluster cannot be deleted if the user first scale down a cluster, and then delete it. The deletion process got stuck

Why

• When the cluster is scaling down, a machine is deleted, with PreTerminateDeleteHookAnnotationPrefix set
• Machine controller set the etcd.k3s.cattle.io/remove to that node, waiting for the etcd.k3s.cattle.io/removed-node-name to be set
• User triggered the deletion to the whole cluster, which deletes other healthy control plane
• Machine controller could not check etcd.k3s.cattle.io/removed-node-name annotation, as the whole cluster is down. And it stuck forever, with the machine not deleted

Fix
Before machine controller remove an etcd member, it will check if the cluster/cp is already under deletion, or if the removed machine is the last node, or if the machine has no noderef. If so, it will skip etcd removal.