Relaxed clippy fixes/nits (#2661)

This PR contains just a few clippy fixes and nits, which are, however,
relaxed by workspace clippy settings here:
https://github.com/paritytech/polkadot-sdk/blob/master/Cargo.toml#L483-L506

---------

Co-authored-by: Dmitry Sinyavin <dmitry.sinyavin@parity.io>
Co-authored-by: ordian <write@reusable.software>
Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <git@kchr.de>

polkadot/xcm/pallet-xcm/src/mock.rs

@@ -28,9 +28,7 @@ use polkadot_parachain_primitives::primitives::Id as ParaId;
 use polkadot_runtime_parachains::origin;
 use sp_core::H256;
 use sp_runtime::{traits::IdentityLookup, AccountId32, BuildStorage};
-pub use sp_std::{
-	cell::RefCell, collections::btree_map::BTreeMap, fmt::Debug, marker::PhantomData,
-};
+pub use sp_std::cell::RefCell;
 use xcm::prelude::*;
 #[allow(deprecated)]
 use xcm_builder::CurrencyAdapter as XcmCurrencyAdapter;