Remove unused load statements in add_type_form.html

This addresses Django 1.5 compatibility as well (see #16)

polymorphic/templates/admin/polymorphic/add_type_form.html

@@ -1,5 +1,5 @@
 {% extends "admin/change_form.html" %}
-{% load i18n admin_modify adminmedia %}
+{% load i18n %}
 {% load url from future %}
 
 {% block breadcrumbs %}{% if not is_popup %}