Visual Studio C # auto-generated method adds "using System"; How do I disable it?

I am using VS Community 2017 Version 15.2

When I use the lightbulb suggestions or manually press (Ctrl +.), It generates a method like this:

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

      

        
        
        
      

    

I want it to be like this:

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

      

        
        
        
      

    

Without:

using System;

      

        
        
        
      

    

I looked through the settings as well as online but couldn't find this option.

I want this because I like to maintain a clean namespace. I don't want to manually delete the using statement every time I use this handy function.