Delete tombstones of the __private module

These are previous names of the `__private` module -- first
`serde::export` then `serde::private` then `serde::__private` -- in all
cases marked `doc(hidden)` and documented as not public API. Leaving a
tombstone made rustc give a better diagnostic "module is private" rather
than "unresolved import". But the rename to `__private` was 2.5 years
ago in dd1f4b4 so it's unlikely anyone
is still benefiting from the tombstone at this point.

serde/src/lib.rs

@@ -307,11 +307,6 @@ pub use crate::ser::{Serialize, Serializer};
 #[path = "private/mod.rs"]
 pub mod __private;
 
-#[allow(unused_imports)]
-use self::__private as export;
-#[allow(unused_imports)]
-use self::__private as private;
-
 #[path = "de/seed.rs"]
 mod seed;