ox.el: Remove docstring for nonexistent argument

* lisp/ox.el (org-export-output-file-name): Remove docstring for
  nonexistent argument VISIBLE-ONLY.

lisp/ox.el

@@ -5905,9 +5905,6 @@ of subtree at point.
 When optional argument PUB-DIR is set, use it as the publishing
 directory.
 
-When optional argument VISIBLE-ONLY is non-nil, don't export
-contents of hidden elements.
-
 Return file name as a string."
   (let* ((visited-file (buffer-file-name (buffer-base-buffer)))
 	 (base-name