Revert "kubelet/network: report but tolerate errors returned from GetNetNS()"

pkg/kubelet/network/plugins.go

@@ -144,8 +144,6 @@ type Host interface {
 // CNI plugin wrappers like kubenet.
 type NamespaceGetter interface {
 	// GetNetNS returns network namespace information for the given containerID.
-	// Runtimes should *never* return an empty namespace and nil error for
-	// a container; if error is nil then the namespace string must be valid.
 	GetNetNS(containerID string) (string, error)
 }