Object: Remove unused category boilerplate

We might want to re-add something like this if/when we find a good use case
for it and do the effort to categorize all objects in the API properly.

Until then, it's better to remove that boilerplate since it's not needed.

Closes #18711.

core/io/stream_peer.h

@@ -39,7 +39,6 @@
 
 class StreamPeer : public RefCounted {
 	GDCLASS(StreamPeer, RefCounted);
-	OBJ_CATEGORY("Networking");
 
 protected:
 	static void _bind_methods();