Delete TODO to use docker client

Re conversation in https://github.com/kubernetes/kubernetes/pull/87373,
we should keep the current behavior (i.e. using the docker binary
instead of the docker client). Delete the TODO instructing us to change
the behavior.

test/e2e_node/image_list.go

@@ -95,7 +95,6 @@ func (dp *dockerPuller) Name() string {
 }
 
 func (dp *dockerPuller) Pull(image string) ([]byte, error) {
-	// TODO(random-liu): Use docker client to get rid of docker binary dependency.
 	return exec.Command("docker", "pull", image).CombinedOutput()
 }