Edit – As noted in the comments, in more recent versions this is now found at Settings > Editor > General > Appearance > Uncheck “Show hard wrap and visual guides”
It’s called the ‘right margin’, and it indicates the width of your code, so that if you are trying to comply with a coding standard that requires a maximum width you can easily see where that is.
To remove it completely go into Settings, and under IDE settings find Editor -> Appearance, then uncheck the box marked “Show right margin”.
Alternatively, if you want to reposition the line, you can do so under Project Settings -> Code Style -> General, and change the Right Margin value.