Aura: Skip initialize block & remove cache (#9132)

This instructs the Aura runtime api to skip initialize block, when
requesting the authorities. This is important, as we don't want to use
the new authorities that should be used from the next block on.
Besides that, it removes the caching stuff. The cache is not available
on full nodes anyway. In the future we should store the authorities
probably in the aux store.

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

@@ -90,6 +90,7 @@ sp_api::decl_runtime_apis! {
 		fn slot_duration() -> SlotDuration;
 
 		// Return the current set of authorities.
+		#[skip_initialize_block]
 		fn authorities() -> Vec<AuthorityId>;
 	}
 }