I ran into this problem and I have not seen this method suggested yet, but I solved it by changing the order of my plotted lines which affects what the legend displays (I am using MATLAB R2017b).
Let's say you plot 2 lines first, and then create a legend. Then you plot a 3rd line. That 3rd line will be added to your legend as 'data 1'.
Instead plot 3 lines, and then in your legend, label just the first 2 lines. The 3rd line will be omitted from the legend.
Hope this helps.