[O] make slim auto-complete work in Org-mode (delete some ac-source in Org-mode).

[stardiviner]

I setup default =ac-sources= for auto-complete like this:

#+BEGIN_SRC emacs-lisp
(setq-default ac-sources
              '(ac-source-yasnippet
                ac-source-abbrev
                ac-source-filename
                ac-source-files-in-current-dir
                ac-source-dictionary
                ac-source-words-in-same-mode-buffers
                ))
#+END_SRC

And I found =ac-source-dictionary= and =ac-source-words-in-same-mode-buffers= is
heavy for Org-mode, So I try to remove them only in Org-mode.

This is my try:

#+BEGIN_SRC emacs-lisp
(add-hook 'org-mode-hook
          (lambda ()
            ;; use `delq' in a loop.
            (mapc (lambda (x) (setq-local ac-sources (delq x ac-sources)))
                  '(ac-source-dictionary 
ac-source-words-in-same-mode-buffers))))
#+END_SRC

But this code will use =delq= to delete *global default* =ac-sources= too. But 
I only
want to delete in Org-mode.
So is there some other ways to do it?