Remove alloc::prelude

As per the libs team decision in rust-lang#58935.

Closes rust-lang#58935

Author: Amanieu

alloc/src/lib.rs

@@ -189,7 +189,6 @@ mod boxed {
 pub mod borrow;
 pub mod collections;
 pub mod fmt;
-pub mod prelude;
 pub mod raw_vec;
 pub mod rc;
 pub mod slice;