function plt_hide_plain_text_custom_post_type_menus() {
//Hide the "Pages → Plain Text Files" menu.
remove_submenu_page('edit.php?post_type=page', 'edit.php?post_type=plain_text_post');
}
add_action('admin_menu', 'plt_hide_plain_text_custom_post_type_menus', 15);