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.
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git-svn-id: http://core.svn.wordpress.org/trunk@27017 1a063a9b-81f0-0310-95a4-ce76da25c4cd