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/runtime/parachains/src/assigner_on_demand/mod.rs

@@ -337,7 +337,12 @@ where
 		ensure!(spot_price.le(&max_amount), Error::<T>::SpotPriceHigherThanMaxAmount);
 
 		// Charge the sending account the spot price
-		T::Currency::withdraw(&sender, spot_price, WithdrawReasons::FEE, existence_requirement)?;
+		let _ = T::Currency::withdraw(
+			&sender,
+			spot_price,
+			WithdrawReasons::FEE,
+			existence_requirement,
+		)?;
 
 		let assignment = Assignment::new(para_id);