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.

cumulus/parachains/runtimes/bridge-hubs/test-utils/src/test_cases.rs

@@ -909,7 +909,6 @@ where
 				]),
 			]),
 		},
-		RefundSurplus,
 		DepositAsset {
 			assets: Wild(All),
 			beneficiary: MultiLocation { parents: 1, interior: X1(Parachain(1000)) },