Using the same `$post_new_file` global variable name in `wp-admin/menu.php` in [33723] was unintentional, but we're already unsetting other variables that are in global scope so we can just unset this one as well. Someday the admin menu won't be a big pile of sad and cryptic globals. Maybe.
props johnjamesjacoby, norcross.
fixes#34045.
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