org-mobile.el: Remove useless defvar

* org-mobile.el: Remove useless defvar.

lisp/org-mobile.el

@@ -307,8 +307,6 @@ Also exclude files matching `org-mobile-files-exclude-regexp'."
 	(push (cons file link-name) rtn)))
     (nreverse rtn)))
 
-(defvar org-agenda-filter)
-
 ;;;###autoload
 (defun org-mobile-push ()
   "Push the current state of Org affairs to the target directory.