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.