libnet: Remove unused cmd/readme_test
**- What I did**

This command was originally added by https://github.com/moby/libnetwork/commit/ea7f555446401de09fc2410aba691bf70bbb2dee to test the code snippet put into libnet's README.md. Nothing compiles this file and it doesn't add any value to the project. So better remove it than maintaining it.

This commit also removes the code snippet from libnet's README.md for the same reasons.
libnetwork/README.md
# libnetwork - networking for containers

Libnetwork provides a native Go implementation for connecting containers

The goal of libnetwork is to deliver a robust Container Network Model that provides a consistent programming interface and the required network abstractions for applications.

#### Design
Please refer to the [design](docs/design.md) for more information.

#### Using libnetwork

There are many networking solutions available to suit a broad range of use-cases. libnetwork uses a driver / plugin model to support all of these solutions while abstracting the complexity of the driver implementations by exposing a simple and consistent Network Model to users.


```go
package main

import (
	"fmt"
	"log"

	"github.com/docker/docker/libnetwork"
	"github.com/docker/docker/libnetwork/config"
	"github.com/docker/docker/libnetwork/netlabel"
	"github.com/docker/docker/libnetwork/options"
)

func main() {
	// Select and configure the network driver
	networkType := "bridge"

	// Create a new controller instance
	driverOptions := options.Generic{}
	genericOption := make(map[string]interface{})
	genericOption[netlabel.GenericData] = driverOptions
	controller, err := libnetwork.New(config.OptionDriverConfig(networkType, genericOption))
	if err != nil {
		log.Fatalf("libnetwork.New: %s", err)
	}

	// Create a network for containers to join.
	// NewNetwork accepts Variadic optional arguments that libnetwork and Drivers can use.
	network, err := controller.NewNetwork(networkType, "network1", "")
	if err != nil {
		log.Fatalf("controller.NewNetwork: %s", err)
	}

	// For each new container: allocate IP and interfaces. The returned network
	// settings will be used for container infos (inspect and such), as well as
	// iptables rules for port publishing. This info is contained or accessible
	// from the returned endpoint.
	ep, err := network.CreateEndpoint("Endpoint1")
	if err != nil {
		log.Fatalf("network.CreateEndpoint: %s", err)
	}

	// Create the sandbox for the container.
	// NewSandbox accepts Variadic optional arguments which libnetwork can use.
	sbx, err := controller.NewSandbox("container1",
		libnetwork.OptionHostname("test"),
		libnetwork.OptionDomainname("example.com"))
	if err != nil {
		log.Fatalf("controller.NewSandbox: %s", err)
	}

	// A sandbox can join the endpoint via the join api.
	err = ep.Join(sbx)
	if err != nil {
		log.Fatalf("ep.Join: %s", err)
	}

	// libnetwork client can check the endpoint's operational data via the Info() API
	epInfo, err := ep.DriverInfo()
	if err != nil {
		log.Fatalf("ep.DriverInfo: %s", err)
	}

	macAddress, ok := epInfo[netlabel.MacAddress]
	if !ok {
		log.Fatal("failed to get mac address from endpoint info")
	}

	fmt.Printf("Joined endpoint %s (%s) to sandbox %s (%s)\n", ep.Name(), macAddress, sbx.ContainerID(), sbx.Key())
}
```

## Contributing

Want to hack on libnetwork? [Docker's contributions guidelines](https://github.com/docker/docker/blob/master/CONTRIBUTING.md) apply.

## Copyright and license
Code and documentation copyright 2015 Docker, inc. Code released under the Apache 2.0 license. Docs released under Creative commons.

# libnetwork - networking for containers

Libnetwork provides a native Go implementation for connecting containers

The goal of libnetwork is to deliver a robust Container Network Model that provides a consistent programming interface and the required network abstractions for applications.

#### Design
Please refer to the [design](docs/design.md) for more information.

#### Using libnetwork

There are many networking solutions available to suit a broad range of use-cases. libnetwork uses a driver / plugin model to support all of these solutions while abstracting the complexity of the driver implementations by exposing a simple and consistent Network Model to users.

## Contributing

Want to hack on libnetwork? [Docker's contributions guidelines](https://github.com/docker/docker/blob/master/CONTRIBUTING.md) apply.

## Copyright and license
Code and documentation copyright 2015 Docker, inc. Code released under the Apache 2.0 license. Docs released under Creative commons.

Yeah, makes sense. Thought I'd mention it as an option before we rip it out, but agreed that it's probably not worth the effort to write new examples.