That line historically represents the margin of A4 paper, which is useful only if you need to print the code. Old school says that you should keep your code inside that margin, to guarantee much portability.
But.. nowadays in the real life, it makes no sense to me! Because we always use advanced editors, bigger displays, and I never needed to print any single line of code.
So, if you want to do it:
Editor -> General -> Appearance -> Show hard wrap guide
Editor -> General -> Appearance -> Show right margin
or
Editor -> Appearance -> Show right margin
To remove line go to:- Preference-> Editor -> General -> Appearance -> Show hard wrap and visual guides. Uncheck it to remove.