Does Article 56 TFEU preclude national tax legislation under which financial institutions not resident in Portuguese territory are subject to tax on interest income received in that territory, withheld at source at the definitive rate of 20% (or at a lower rate if there is an agreement to avoid double taxation), a tax applied to gross income with no possibility of deducting business expenses directly related to the financial activity carried out, whereas the interest received by resident financial institutions is incorporated in the total taxable income, with deduction of any expenses related to the activity pursued when determining the profit for the purposes of corporation tax, so that the basic rate of 25% is applied to the net interest income?