There should be this button at the top left:

If clicking that isn't enough to make it go away, there should be a settings button somewhere there either at the bottom or one you can select by pressing the header and going to the sidebar settings. There you should be able to disable some stuff too.

Hmm, for me it works to click on that sidebar button once to hide the sidebar. And then right-click on the sidebar button and select "Remove from Toolbar".