Remove http/tls mux from etcd client listener

[brandond]

Upstream etcd has seen issues with grpc muxed with http on the client port, and recommends against it.

• Ref: https://github.com/etcd-io/etcd/issues/15402
• Ref: https://github.com/k3s-io/k3s/issues/8415

The http/https server was only used to serve a canned version response which kubeadm used to validate, but it no longer does. Metrics are served over http on a separate port.