LONDON (Reuters) - The Aviva Staff Pension Scheme has completed a 1.7 billion pounds ($2.19 billion) de-risking deal with Aviva Life & Pensions , covering around 5,800 members, the company said on Thursday.The so-called 'buy-in' deal will help protect the defined benefit, or final salary, payments of around 4,300 workers and 1,500 pensioners by insuring against the risk of investment returns falling or people living longer than expected.($1 = 0.7756 pounds)