UPnPContext: remove unused 'using' statement

Change-Id: Ib7f3ae5a49ee73e4c3c9c97b7ca1e2788d241bf3
diff --git a/include/upnp/upnp_context.h b/include/upnp/upnp_context.h
index 11f7e29..ead135f 100644
--- a/include/upnp/upnp_context.h
+++ b/include/upnp/upnp_context.h
@@ -45,8 +45,6 @@
 
 #include <cstdlib>
 
-using random_device = dht::crypto::random_device;
-
 using IgdFoundCallback = std::function<void()>;
 
 namespace dhtnet {