Deleting annotations is as easy as making them. In this instance, the annotation is the light blue highlighted text.
To remove the annotation, hold the mouse over the annotation. The annotation should darken in colour, and a pop-up box should appear.
Click the "X" in the pop up box, to remove the annotation.
Alternatively, if you notice you've made a mistake right away, and want to remove the annotation, push Control + Z on your keyboard, and your last annotation made will be deleted.