Install Stylus or another css extension and then add this style filtered to news.ycombinator.com:

  .pagetop {
      visibility:collapse;
  }
  
  .pagetop * {
      visibility:visible;
  }
  
  .score {
      visibility:hidden;
  }

This would be a great HN feature actually, fitting in spirit with the existing settings (eg showdead etc). @dang, if you read this, what do you think?

If you use an adblocker like uBlock Origin, you can use the following filter to remove the top navbar:

news.ycombinator.com##.pagetop