Do not save target buffer after archiving subtree

* lisp/org-archive.el: (org-archive-subtree) Do not save buffer after
  each archive.  Saving the archive buffer after archiving each
  subtree results in substantial slowdown and many writes to disk when
  archiving an active region.  This brings the behavior of
  org-archive-subtree into line with org-refile, which does not save
  the target buffer after refiling.

lisp/org-archive.el

@@ -381,10 +381,7 @@ direct children of this heading."
 		     (point)
 		     (concat "ARCHIVE_" (upcase (symbol-name item)))
 		     value))))
-	      (widen)
-	      ;; Save and kill the buffer, if it is not the same
-	      ;; buffer.
-	      (unless (eq this-buffer buffer) (save-buffer)))))
+	      (widen))))
 	;; Here we are back in the original buffer.  Everything seems
 	;; to have worked.  So now run hooks, cut the tree and finish
 	;; up.