Art. 918. Disinheritance without a specification of the cause, or for a cause the truth of which, if contradicted, is not proved, or which is not one of those set forth in this Code, shall annul the institution of heirs insofar as it may prejudice the person disinherited; but the devises and legacies and other testamentary dispositions shall be valid to such extent as will not impair the legitime.