Nav Menu: Remove post/page items from the Nav Menu when the post/page is deleted.

This was broken through a change in [25163]. `_menu_item_object` in wp_get_associated_nav_menu_items() is not relevant for post types.
Adds unit tests.

props UmeshSingla for initial patch.
fixes #26795.

git-svn-id: https://develop.svn.wordpress.org/trunk@27150 602fd350-edb4-49c9-b593-d223f7449a82