Implement abstract class fixer adds `static` and doesn't add `override` modifier for an assignment operator

```
abstract class C1
{
    abstract public void operator ++();

    abstract public void operator -=(int i);
}


class C2 : C1
{
    public static void operator -=(int i)
    {
        throw new NotImplementedException();
    }

    public static void operator ++()
    {
        throw new NotImplementedException();
    }
}
```