Fixes import path in benchmark macro (#2437)

Fully-qualified path was not being used in benchmark macro for one of
the cases. This PR fixes this and removes the unnecessary import in a
couple of files, which I believe was done to fix this issue.

substrate/bin/node-template/pallets/template/src/benchmarking.rs

@@ -1,7 +1,6 @@
 //! Benchmarking setup for pallet-template
 #![cfg(feature = "runtime-benchmarks")]
 use super::*;
-use sp_std::vec;
 
 #[allow(unused)]
 use crate::Pallet as Template;