xcm: SovereignPaidRemoteExporter: remove unused RefundSurplus instruction (#2312)

Refunding surplus happens anyway on xcm_executor::post_process(),
automatically refunding surplus to original_origin at the end of
execution. Since SovereignPaidRemoteExporter doesn't ClearOrigin, it can
simply rely on the automatic mechanism.

Furthermore, RefundSurplus instruction refunds _surplus_. Surplus exists
only as a result of Transact, SetErrorHandler or SetAppendix
instructions, none of which being part of the
SovereignPaidRemoteExporter XCM program. So surplus is always zero here
anyway.

polkadot/xcm/xcm-builder/src/universal_exports.rs

@@ -305,7 +305,6 @@ impl<Bridges: ExporterFor, Router: SendXcm, UniversalLocation: Get<InteriorMulti
 				WithdrawAsset(fees.clone().into()),
 				BuyExecution { fees, weight_limit: Unlimited },
 				export_instruction,
-				RefundSurplus,
 				DepositAsset { assets: All.into(), beneficiary: local_from_bridge },
 			]
 		} else {