Remove dm.no_warn_on_loop_devices in info warning

I think we doesn't provide dm.no_warn_on_loop_devices option
at all. I didn't found any code to handle this option.

Signed-off-by: Lei Jitang <leijitang@huawei.com>

api/client/system/info.go

@@ -48,7 +48,7 @@ func runInfo(dockerCli *client.DockerCli) error {
 
 			// print a warning if devicemapper is using a loopback file
 			if pair[0] == "Data loop file" {
-				fmt.Fprintln(dockerCli.Err(), " WARNING: Usage of loopback devices is strongly discouraged for production use. Either use `--storage-opt dm.thinpooldev` or use `--storage-opt dm.no_warn_on_loop_devices=true` to suppress this warning.")
+				fmt.Fprintln(dockerCli.Err(), " WARNING: Usage of loopback devices is strongly discouraged for production use. Use `--storage-opt dm.thinpooldev` to specifies a custom block storage device.")
 			}
 		}
 

docs/userguide/storagedriver/device-mapper-driver.md

@@ -440,7 +440,7 @@ Storage Driver: devicemapper
  Deferred Deletion Enabled: false
  Deferred Deleted Device Count: 0
  Data loop file: /var/lib/docker/devicemapper/devicemapper/data
- WARNING: Usage of loopback devices is strongly discouraged for production use. Either use `--storage-opt dm.thinpooldev` or use `--storage-opt dm.no_warn_on_loop_devices=true` to suppress this warning.
+ WARNING: Usage of loopback devices is strongly discouraged for production use. Use `--storage-opt dm.thinpooldev` to specifies a custom block storage device.
  Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata
  Library Version: 1.02.90 (2014-09-01)
 Logging Driver: json-file