Remove unused code in runtime/polkadot/src/lib.rs (#7540)

* remove SetStorageVersions runtime upgrade

* remove unused imports

runtime/polkadot/src/lib.rs

@@ -1488,35 +1488,12 @@ pub type Migrations = migrations::Unreleased;
 #[allow(deprecated, missing_docs)]
 pub mod migrations {
 	use super::*;
-	use frame_support::traits::{GetStorageVersion, OnRuntimeUpgrade, StorageVersion};
 
 	/// Unreleased migrations. Add new ones here:
 	pub type Unreleased = (
 		pallet_im_online::migration::v1::Migration<Runtime>,
 		parachains_configuration::migration::v7::MigrateToV7<Runtime>,
 	);
-
-	/// Migrations that set `StorageVersion`s we missed to set.
-	pub struct SetStorageVersions;
-
-	impl OnRuntimeUpgrade for SetStorageVersions {
-		fn on_runtime_upgrade() -> Weight {
-			// `Referenda` pallet was added on chain after the migration to version `1` was added.
-			// Thus, it never required the migration and we just missed to set the correct `StorageVersion`.
-			let storage_version = Referenda::on_chain_storage_version();
-			if storage_version < 1 {
-				StorageVersion::new(1).put::<Referenda>();
-			}
-
-			// Was missed as part of: `runtime_common::session::migration::ClearOldSessionStorage<Runtime>`.
-			let storage_version = Historical::on_chain_storage_version();
-			if storage_version < 1 {
-				StorageVersion::new(1).put::<Historical>();
-			}
-
-			RocksDbWeight::get().reads_writes(2, 2)
-		}
-	}
 }
 
 /// Unchecked extrinsic type as expected by this runtime.