Ilja’s solution is only working for Chrome “normal mode” in my case unfortunately, but not in chrome incognito mode, and not with firefox.
I have no clue why again I see this damned Paypal express checkout button.
The class of the checkout buttons in the cart is “cart__additional_checkout”. So I hid it with css. I checked my cart page with “inspect” on firefox, the div is there but hidden, as intended.
.cart__additional_checkout{
display: none !important;
}