The TV Rakharo actually started off as Jhogo, but they changed his name as they felt it was a little too close to 'Drogo' when pronounced with the accent. Then the TV Rakharo ended up as a composite of Jhogo and Rakharo in the books.

Then Elyes Gabel (who's a reasonably well-know face on UK TV, with long-running roles in soaps and a few dramatic roles) was offered a part on the movie version of WORLD WAR Z, working with Brad Pitt for a lot of money, so he decided to do that instead. Apparently HBO gave him their blessing and wrote him out in this fashion (and he was able to take the time off the movie to film an exit scene).

So now the Jhogo/Rakharo story functions will be carried out by Kovarro (whom we saw last week for a few minutes) instead. I'm waiting for him to be cast in another movie and they have to substitute another Dothraki next season. It's like Murphy Brown's secretaries, only if they were skilled barbarian horse warriors.