org-attach: Fix handling of ATTACH tag at sychronize

* lisp/org-attach.el (org-attach-sync): Remove ATTACH tag at
  synchronize when no files left.

lisp/org-attach.el

@@ -507,7 +507,7 @@ This can be used after files have been added externally."
   (let ((attach-dir (org-attach-dir)))
     (when attach-dir
       (let ((files (org-attach-file-list attach-dir)))
-	(and files (org-attach-tag))
+	(org-attach-tag (not files))
 	(when org-attach-file-list-property
 	  (dolist (file files)
 	    (unless (string-match "^\\.\\.?\\'" file)