How do I remove search bar that isn’t a widget?

You can that by css :

1. Appearance -> Customize -> Additional CSS

2. Paste this code AFTER all the contents

   .main-navigation .menu-item-search {
   display: none;
   }

3. Save

You can also do that by adding this code of line in the bottom most part of the functions.

1. Login to Dashboard
2. Go to Appearance -> Editor
3. Click Theme Functions at the right side

4. Add code at the last part after all the content

   remove_filter( 'wp_nav_menu_items', 'divina_add_search_box_to_divina_menu', 10, 2 );
   

5. Save.