Provide `Amount` instead of `OutputValue` to `FillOrder` command

[azarovh]

Providing OutputValue (i.e. currency + amount) to the AccountCommand::FillOrder is redundant. Order can be filled with only particular currency defined on creation by OrderData. As a result check for currency mismatch can be removed from orders-accounting, making it one rule less in the code.

At the moment this change can be done without a fork because orders are not activated.