The Secondary Navigation displays subpages for a page. You cannot remove individual links from it.
This article includes instructions to:
Unpublish the page that you do not want to appear in the menu.
Note:
This completely removes access to the page throughout the site.
Hide the page if you do not want it to appear in the menu.
Note:
This hides the page while keeping it published. It does not appear in menus or search results. Users need a direct link to view the page.
Note:
Pages still appear in Search and all other menus. You can choose which pages appear in this page's Secondary Navigation.