Fix a compilation warning

Author: tdimitrov

substrate/frame/staking/src/migrations.rs

@@ -25,8 +25,6 @@ use frame_support::{
 	traits::{GetStorageVersion, OnRuntimeUpgrade, UncheckedOnRuntimeUpgrade},
 };
 
-#[cfg(feature = "try-runtime")]
-use frame_support::ensure;
 #[cfg(feature = "try-runtime")]
 use sp_runtime::TryRuntimeError;