This:

Ctrl + Alt + left_arrow

If you have multiple split screens, hit the above to get rid of them.

It's quite annoying when you accidentally split the screen, but more so when there's no obvious way to get rid of it! Perhaps there needs to be a split down button next to the split screen button to make it easier.

Until then, the above works a treat. Conversely, Ctrl + Alt + right_arrow if you want to split.