You cannot "get rid of the line" because you always have at least 1 stage you are working in. If you only want to work with one stage, you can drag an element just before the line to push the stage line further out. 

You can also scroll as far right as you'd like and drop a new element in a new stage. This will expand the canvas as far as the element was dragged. Then click Delete on that new stage, and choose the option to Merge objects into the previous stage. You'll have the single stage but it will be much wider.