Can't find a link and my memory is a bit sketchy, but I think the Government have just created a London Policing Committee, which will duplicate most of the functions of the Metropolitan Police Authority, but probably have more power within the Government to influence change.
Boris has bailed out as Chairman of the MPA because being an ex-MP, he knows that it has now been stripped of all its powers.
Quite why the Government didn't undermine the MPA during Ken's reign is a mystery.