I'm not sure if it can be removed, I had a bit of a look but couldn't really find an easy way.

But if you do accidentally click on it just draw a large square over as much of the preview window as you can and the exposure will be adjusted pretty evenly. If you find it has blown out or overly darkened your preview, or you want to just remove the adjustment you have made bey accident you can always go to the Draw Style window and using the drop down select Restore Render Setting Defaults which sets every back to how they render by default.