Do not apply discount at get_invoice_line()

[lukio]

Is not necesary to apply discount there, because the unit_price is the number as we wanted.
Also, the unit_price changes if the period of that line is just for a few days, so is dificult to know which discount must be applied.

We also remove the contract_start_date and contract_end_date (discounts dates), because we use the start and end date that came from contract module.