xds/cdsbalancer: stop handling subconn state updates

[dfawley]

Because its child, clusterresolver, uses StateListener always (

grpc-go/xds/internal/balancer/clusterresolver/clusterresolver.go

Lines 424 to 429 in e9a4e94

	if opts.StateListener == nil {
	// If already set, just allow updates to be sent directly to the
	// child's listener. Otherwise, we are responsible for forwarding the
	// update we'll receive to the proper child.
	opts.StateListener = func(state balancer.SubConnState) { c.b.UpdateSubConnState(sc, state) }
	}

), this change is safe to make in isolation now.

RELEASE NOTES: none