I think Kalanick has a majority of Uber's voting shares, so neither the board nor anyone else can force him to leave. Likewise, if Kalanick wants Levandowski to stay, he stays.
I don't think that works if there is truly criminal culpability here. If you're on the Board of Directors you can fire the clown or you can let him vote you off the board. You don't get to say: I can't do anything because he's the majority shareholder.
How do you fire any one as a board member? Your only input is the vote and if the CEO control the majority of votes, your vote is meaningless.
It's not always that simple though. Kalanick can't just do whatever he wants with no fear of other shareholders intervening.