It’s called the Line Missile Display and you can Disable it when you are in a game by
1) pressing ESC
2) then go to Options
3) then Turn off the Line Missile Display checkbox
This will restore the original targeting system which is just a big circle that shows you the range of the skillshot.
Changing to the original system makes your skillshots 100 times more accurate because its easier to control a crosshair which moves predictably with your mouse movement than a directional arrow which moves at different speeds depending on the distance your cursor is from your champion.