chore(org-mode): switch to built-in org-mode

The upstream org-mode package seems to have changes on a daily basis,
and org-mode is one of the slowest packages to install. This should
avoid those long wait times.

modules/org-mode/siren-org-mode.el

@@ -13,6 +13,7 @@
 (require 'siren-smartparens)
 
 (use-package org
+  :straight (:type built-in)
   :bind
   (:map org-mode-map
         ("C-j" . org-return-indent)