Coding Standards: Remove unnecessary isset() check in wp-admin/post.php.

`wp_die()` is called a few lines earlier if `$post_type_object` is falsey.

Follow-up to [12597], [17169], [24201].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60344 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/post.php

@@ -160,7 +160,7 @@
 			$submenu_file  = 'upload.php';
 			$post_new_file = 'media-new.php';
 		} else {
-			if ( isset( $post_type_object ) && $post_type_object->show_in_menu && true !== $post_type_object->show_in_menu ) {
+			if ( $post_type_object->show_in_menu && true !== $post_type_object->show_in_menu ) {
 				$parent_file = $post_type_object->show_in_menu;
 			} else {
 				$parent_file = "edit.php?post_type=$post_type";