Eliminate experimental feature (#3654)

Totally removes the `experimental` feature. Closes #3648.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/frame/aura/src/lib.rs

@@ -66,9 +66,6 @@ const LOG_TARGET: &str = "runtime::aura";
 ///
 /// This was the default behavior of the Aura pallet and may be used for
 /// backwards compatibility.
-///
-/// Note that this type is likely not useful without the `experimental`
-/// feature.
 pub struct MinimumPeriodTimesTwo<T>(sp_std::marker::PhantomData<T>);
 
 impl<T: pallet_timestamp::Config> Get<T::Moment> for MinimumPeriodTimesTwo<T> {
@@ -117,10 +114,6 @@ pub mod pallet {
 		/// The effective value of this type should not change while the chain is running.
 		///
 		/// For backwards compatibility either use [`MinimumPeriodTimesTwo`] or a const.
-		///
-		/// This associated type is only present when compiled with the `experimental`
-		/// feature.
-		#[cfg(feature = "experimental")]
 		type SlotDuration: Get<<Self as pallet_timestamp::Config>::Moment>;
 	}
 
@@ -250,17 +243,7 @@ impl<T: Config> Pallet<T> {
 
 	/// Determine the Aura slot-duration based on the Timestamp module configuration.
 	pub fn slot_duration() -> T::Moment {
-		#[cfg(feature = "experimental")]
-		{
-			T::SlotDuration::get()
-		}
-
-		#[cfg(not(feature = "experimental"))]
-		{
-			// we double the minimum block-period so each author can always propose within
-			// the majority of its slot.
-			<T as pallet_timestamp::Config>::MinimumPeriod::get().saturating_mul(2u32.into())
-		}
+		T::SlotDuration::get()
 	}
 
 	/// Ensure the correctness of the state of this pallet.

substrate/frame/aura/src/mock.rs

@@ -84,8 +84,6 @@ impl pallet_aura::Config for Test {
 	type DisabledValidators = MockDisabledValidators;
 	type MaxAuthorities = ConstU32<10>;
 	type AllowMultipleBlocksPerSlot = AllowMultipleBlocksPerSlot;
-
-	#[cfg(feature = "experimental")]
 	type SlotDuration = ConstU64<SLOT_DURATION>;
 }