Use docker api IsErrNotFound(). String handling is not required anymore

Why? https://github.com/kubernetes/kubernetes/pull/75843#discussion_r273785877
Moby support: https://github.com/moby/moby/pull/38689
Moby def: 053c6f097a/client/errors.go (L44-L49)

pkg/kubelet/dockershim/libdocker/helpers.go

@@ -161,12 +161,3 @@ func matchImageIDOnly(inspected dockertypes.ImageInspect, image string) bool {
 	klog.V(4).Infof("The reference %s does not directly refer to the given image's ID (%q)", image, inspected.ID)
 	return false
 }
-
-// isImageNotFoundError returns whether the err is caused by image not found in docker
-// TODO: Use native error tester once ImageNotFoundError is supported in docker-engine client(eg. ImageRemove())
-func isImageNotFoundError(err error) bool {
-	if err != nil {
-		return strings.Contains(err.Error(), "No such image:")
-	}
-	return false
-}

pkg/kubelet/dockershim/libdocker/kube_docker_client.go

@@ -399,7 +399,7 @@ func (d *kubeDockerClient) RemoveImage(image string, opts dockertypes.ImageRemov
 	if ctxErr := contextError(ctx); ctxErr != nil {
 		return nil, ctxErr
 	}
-	if isImageNotFoundError(err) {
+	if dockerapi.IsErrNotFound(err) {
 		return nil, ImageNotFoundError{ID: image}
 	}
 	return resp, err