MDL-56275 blog: do not display the 'choose a link' button

When registereing an external blog the choose a link button is
misleading, we don't have any repos which restrict to RSS feeds, so the
majority of results you get from the repo picker will be wrong.

blog/external_blog_edit_form.php

@@ -36,7 +36,7 @@ public function definition() {
 
         $mform =& $this->_form;
 
-        $mform->addElement('url', 'url', get_string('url', 'blog'), array('size' => 50));
+        $mform->addElement('url', 'url', get_string('url', 'blog'), array('size' => 50), array('usefilepicker' => false));
         $mform->setType('url', PARAM_URL);
         $mform->addRule('url', get_string('emptyurl', 'blog'), 'required', null, 'client');
         $mform->addHelpButton('url', 'url', 'blog');