Annoying "No Custom Tools Available" notification won't go away,

I have this notification window in the Scene Editor window that won’t go away,

No custom tools availible

It’s still there after restarting Unity. I’ve tried pressing escape, clicking on it, dragging it, right clicking on it and shouting at it.

How do I make this go away?

The documentation say this appears it I’ve clicked on the tools icon and there aren’t any custom tools (which I’m sure I have done). Surely is should vanish after a while though.

As per

There’s a second custom tool button on the Editor Window:

Custom Tool Icon