Move parent subchannel construction args...

... into grpc_channel itself, to hide this complicated construction
detail.
diff --git a/src/core/client_config/subchannel_factory.h b/src/core/client_config/subchannel_factory.h
index aae47cc..854d2e2 100644
--- a/src/core/client_config/subchannel_factory.h
+++ b/src/core/client_config/subchannel_factory.h
@@ -44,12 +44,6 @@
 };
 
 struct grpc_subchannel_args {
-  /* TODO(ctiller): consider making (parent, metadata_context) more opaque
-     - these details are not needed at this level of API */
-  /** Parent channel element - passed from the master channel */
-  grpc_channel_element *parent;
-  /** Metadata context for this channel - passed from the master channel */
-  grpc_mdctx *metadata_context;
   /** Channel filters for this channel - wrapped factories will likely
       want to mutate this */
   const grpc_channel_filter **filters;
@@ -70,10 +64,9 @@
 
 void grpc_subchannel_factory_ref(grpc_subchannel_factory *factory);
 void grpc_subchannel_factory_unref(grpc_subchannel_factory *factory);
+
 /** Create a new grpc_subchannel */
 void grpc_subchannel_factory_create_subchannel(grpc_subchannel_factory *factory,
                                                grpc_subchannel_args *args);
 
-grpc_subchannel_factory *grpc_default_subchannel_factory();
-
 #endif /* GRPC_INTERNAL_CORE_CLIENT_CONFIG_SUBCHANNEL_FACTORY_H */