babel: Use `copy-sequence' instead of `copy-tree'

* lisp/ob.el: `copy-sequence' suffices to copy alist; no need for
`copy-tree'

lisp/ob.el

@@ -611,7 +611,7 @@ the current subtree."
   (let ((print-level nil)
 	(info (or info (org-babel-get-src-block-info))))
     (setf (nth 2 info)
-	  (sort (copy-tree (nth 2 info))
+	  (sort (copy-sequence (nth 2 info))
 		(lambda (a b) (string< (car a) (car b)))))
     (let ((hash (sha1
 		 (format "%s-%s"