Revert "org.el: Include `org-tag-alist' in the list for tag completions."

This reverts commit acc7a0b2bbb440b89039d56a3ffdd4bb17c9bf47.

See http://article.gmane.org/gmane.emacs.orgmode/54406

lisp/org.el

@@ -13424,8 +13424,7 @@ With prefix ARG, realign all tags in headings in the current buffer."
 	;; Get a new set of tags from the user
 	(save-excursion
 	  (setq table (append org-tag-persistent-alist
-			      org-tag-alist
-			      (org-get-buffer-tags)
+			      (or org-tag-alist (org-get-buffer-tags))
 			      (and
 			       org-complete-tags-always-offer-all-agenda-tags
 			       (org-global-tags-completion-table