At the point, there cane only two possibilities
There are already secret criminal investigations already in process or Sessions needs to go.
Knowing what we know of Trump, I assert it is impossible for him to be held hostage by Sessions, like some impotent wimp.
For one, Trumps personality indicates he simply wouldn't tolerate this kind of insubordination, even in matters of no consequence. To the contrary, the DOJ prosecution of treason is of utmost importance to Trump and the country.
For two, for those who hold that firing Sessions would leave Rosenstein in charge, well, he can be fired as well. What's that you say? The Senate has signaled they won't confirm another AG. I thought Trump was a genius, working 20 hours a day. What's to prevent him from devoting 8 hours per day to the AG slot? Why couldn't he hire Rudy et al as his personal consultant?
You see, as you work through the possibilities, and add in Trump's personality, background and road to the presidency, nothing that suggests Session is atually frustrating his goals & directions makes any sense. Add to the fact that deep state treason well & truly represents an existential threat to Trump, his family and the republic (as in, they'll eliminate him if he doesn't succeed), and well, it just doesn't add up.
Lastly, even if he was somehow in a checkmate situation, would someone like Trump publicly reveal this situation and embarrass himself in front of the entire world with these pathetic tweets? Who, never mind Trump, pathetically debases himself in such a fashion?
Nope, eliminate the impossible and what you're left holding is logic.