Fix two minor warnings (#6650)

* Unused import in no_std builds
* Global attribute in non-root of a crate

Author: athei

src/benchmarking.rs

@@ -17,8 +17,6 @@
 
 //! Benchmarks for the BABE Pallet.
 
-#![cfg_attr(not(feature = "std"), no_std)]
-
 use super::*;
 use frame_benchmarking::benchmarks;