Move stake_weighted_timestamp module (#18114) (#18120)

* Move timestamp module into runtime

* Less public

* Remove unused enum

(cherry picked from commit 19fe1dd463)

Co-authored-by: Tyera Eulberg <teulberg@gmail.com>

sdk/src/lib.rs

@@ -44,7 +44,6 @@ pub mod secp256k1_instruction;
 pub mod shred_version;
 pub mod signature;
 pub mod signer;
-pub mod stake_weighted_timestamp;
 pub mod system_transaction;
 pub mod timing;
 pub mod transaction;