Rollup merge of #134272 - RalfJung:destabilize-rustc_encodable_decodable, r=oli-obk

Remove rustc_encodable_decodable feature

This has been shown in future-compat reports since Rust 1.79 (https://github.com/rust-lang/rust/pull/116016), released June 2024. Let's see if crater still finds any issues.

Part of https://github.com/rust-lang/rust/issues/134301.

Cc ``@rust-lang/libs-api``

library/core/src/prelude/mod.rs

@@ -18,16 +18,6 @@ mod common;
 pub mod v1 {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::common::*;
-
-    // Do not `doc(inline)` these `doc(hidden)` items.
-    #[unstable(
-        feature = "rustc_encodable_decodable",
-        issue = "none",
-        soft,
-        reason = "derive macro for `rustc-serialize`; should not be used in new code"
-    )]
-    #[allow(deprecated)]
-    pub use crate::macros::builtin::{RustcDecodable, RustcEncodable};
 }
 
 /// The 2015 version of the core prelude.