Remove duplicate `nat`, `null` in `docker info` for Windows

This fix tries to address the issue raised in 27695 where
duplicate `nat` and `null` has been listed in `docker info`
for Windows.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

daemon/network.go

@@ -364,6 +364,9 @@ func (daemon *Daemon) GetNetworkDriverList() []string {
 
 	pluginList := daemon.netController.BuiltinDrivers()
 	pluginMap := make(map[string]bool)
+	for _, plugin := range pluginList {
+		pluginMap[plugin] = true
+	}
 
 	networks := daemon.netController.Networks()