PSBT input/output Merge() methods always return true unconditionally and have no failure paths. As a result, the return value checks in PartiallySignedTransaction::Merge() can never fail and are dead code.
This makes the bool return type and [[nodiscard]] misleading.
This PR changes both methods to return void and remove the return value checks. If at some point in the future the failure logic is needed, it can be introduced again. For now using void makes the current behavior easier to understand.
