Avoid auto generated namespace in Visual Studio. How to turn off automatic insertion of "using System;"

I am using VS Community 2017 Version 15.2

When I use the light-bulb suggestions, or manually press (Ctrl + .), it generates a method like this:

using System;
public void f() {
    throw new NotImplementedException();
}

I want it to be this way:

public void f() {
    throw new System.NotImplementedException();
}

Without the:

using System;

I looked through the settings and also online but could not find the option.

I want this because I like keeping my namespace clean. I don't want to manually delete the using statement every time I use this handy feature.