Revert "org.el: Fix bug when gathering tag completion options."

This reverts commit e48d67ed6d6a0f352449fbeea6091b6f216c9990.

This commit doesn't take care of tag groups.

lisp/org.el

@@ -13730,10 +13730,7 @@ Returns the new tags string, or nil to not change the current settings."
 				 (append (or buffer-tags
 					     (with-current-buffer buf
 					       (mapcar 'car (org-get-buffer-tags))))
-					 (delq nil
-					       (mapcar (lambda (x)
-							 (if (stringp
-							      (car x)) x)) table))))))
+					 (mapcar 'car table)))))
 		    (quit (setq tg "")))
 		  (when (string-match "\\S-" tg)
 		    (add-to-list 'buffer-tags (list tg))