std: Remove old_io/old_path from the prelude

This commit removes the reexports of `old_io` traits as well as `old_path` types
and traits from the prelude. This functionality is now all deprecated and needs
to be removed to make way for other functionality like `Seek` in the `std::io`
module (currently reexported as `NewSeek` in the io prelude).

Closes #23377
Closes #23378

src/libserialize/json.rs

@@ -2622,6 +2622,7 @@ mod tests {
     use std::{i64, u64, f32, f64};
     use std::collections::BTreeMap;
     use std::string;
+    use std::old_io::Writer;
 
     #[derive(RustcDecodable, Eq, PartialEq, Debug)]
     struct OptionData {