On your Secondary Navigation, which displays subpages on a page, you cannot individually remove links.
You can:
Unpublish the page that you do not want to display on the menu.
Note:
This will completely remove page access across the entire site.
Hide the page that you do not want to display on the menu.
Note:
This will hide the page. The page will still be published, but it will not appear in menus or search results. Users can only view the page if they have a direct link.
Note:
Pages will still appear in search and all other menus. However, you can choose which pages display in this page’s secondary navigation.