The “old” Donald Trump would have already fired Sessions but, we have a “new” Donald, the kinder, genteeler, Donald. He WILL eventually give Sessions the option to gracefully step down to save his reputation or, at worst be fired. So far that option doesn't seem to been offered, at least not yet.
I think Trump is not firing anybody because the more the wheels come off of this scam in a very public way, the more it is helping Trump.