xcm: SovereignPaidRemoteExporter: remove useless RefundSurplus instruction

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 anyway.