Delete :grouptags from tags list when creating org-mobile index file

* org-mobile.el (org-mobile-create-index-file): delete :grouptags
  entries from tags list when creating the org-mobile index file.

Reported by Dror Atariah - see

    http://thread.gmane.org/gmane.emacs.orgmode/79803

and (the continuation of the above thread)

    http://thread.gmane.org/gmane.emacs.orgmode/79856

lisp/org-mobile.el

@@ -459,6 +459,7 @@ agenda view showing the flagged items."
 			      ((stringp x) x)
 			      ((eq (car x) :startgroup) "{")
 			      ((eq (car x) :endgroup) "}")
+			      ((eq (car x) :grouptags) nil)
 			      ((eq (car x) :newline) nil)
 			      ((listp x) (car x))))
 		      def-tags))