consensus-slots: cleanup SlotDuration config (#10878)

* consensus-slots: cleanup the SlotDuration config

* fix tests

* address review comments

substrate/primitives/consensus/babe/src/lib.rs

@@ -79,7 +79,7 @@ pub const MEDIAN_ALGORITHM_CARDINALITY: usize = 1200; // arbitrary suggestion by
 /// The index of an authority.
 pub type AuthorityIndex = u32;
 
-pub use sp_consensus_slots::Slot;
+pub use sp_consensus_slots::{Slot, SlotDuration};
 
 /// An equivocation proof for multiple block authorships on the same slot (i.e. double vote).
 pub type EquivocationProof<H> = sp_consensus_slots::EquivocationProof<H, AuthorityId>;
@@ -237,13 +237,6 @@ impl AllowedSlots {
 	}
 }
 
-#[cfg(feature = "std")]
-impl sp_consensus::SlotData for BabeGenesisConfiguration {
-	fn slot_duration(&self) -> std::time::Duration {
-		std::time::Duration::from_millis(self.slot_duration)
-	}
-}
-
 /// Configuration data used by the BABE consensus engine.
 #[derive(Clone, PartialEq, Eq, Encode, Decode, RuntimeDebug, MaxEncodedLen, TypeInfo)]
 #[cfg_attr(feature = "std", derive(Serialize, Deserialize))]