MAINT: Reduce number of parameters for create_transaction.

create_transaction accepted both sid and order, which in all cases
was derived from the current event, so remove `sid` and `order`,
replacing them with event

If there is a scenario where sid and order need to be set independently
of each other, then the underlying Transaction object can be called
directly.

Looking towards making writing custom slippage models slightly easier
by removing the redundancy.