Migrate slots pallet to pallet attribute macro. (#3218)

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

polkadot/runtime/common/src/integration_tests.rs

@@ -28,7 +28,6 @@ use sp_keystore::{KeystoreExt, testing::KeyStore};
 use primitives::v1::{BlockNumber, Header, Id as ParaId, ValidationCode, HeadData, LOWEST_PUBLIC_ID};
 use frame_support::{
 	parameter_types, assert_ok, assert_noop, PalletId,
-	storage::StorageMap,
 	traits::{Currency, OnInitialize, OnFinalize, KeyOwnerProofSystem},
 };
 use frame_system::EnsureRoot;