The prime Attunement buff is a buff provided by the game system to allow Prime Engrams to drop in the world, and it is not granted by any specific NPC. The game allows someone to get a certain number of Prime drops over a certain amount of time (the number and time are values I don't specifically know). Once you have reached the allotment of Prime drops for the time frame the game is checking, the buff will be removed. The buff will come back once the game determines you're now below the number you should be for the time frame it checks, thus allowing you to get Prime drops once more. There is nothing you have to do to make the buff show up or go away outside of playing the game.