how to remove the top navigation bar or header top on wordpress website?

Looks like the author didn’t include the check for menu items before calling the top nav bar.

Two methods:

1. Dashboard –> appearance –> customize –> Additional CSS add the following piece of code and save changes:

   #topnav {
     display:none;
   }
   

2. Create a child theme and include this page neville\template-parts\partials\headers\header-tmpl-default.php. Make sure to remove the below line from the page you included in the child theme:

   add_action( 'neville__header_default', neville__header_default_top_nav',      20 );
   

Either one should work. However, I’d recommend the second method. Its always better to have a child theme while making changes to any theme.