fix sp-timestamp warn

Author: pgherveou

Cargo.lock

@@ -67,7 +67,6 @@ sp-runtime.default-features = true
 sp-runtime.workspace = true
 sp-statement-store.default-features = true
 sp-statement-store.workspace = true
-sp-timestamp.workspace = true
 substrate-frame-rpc-system.default-features = true
 substrate-frame-rpc-system.workspace = true
 substrate-state-trie-migration-rpc.default-features = true