MDL-37621 assign: Fix php error on create new assign

I removed one too many defaults in the previous patch. This patch
puts back the default for teamsubmissiongroupingid.

mod/assign/mod_form.php

@@ -153,6 +153,7 @@ public function definition() {
         $mform->addElement('select', 'teamsubmissiongroupingid', $name, $options);
         $mform->addHelpButton('teamsubmissiongroupingid', 'teamsubmissiongroupingid', 'assign');
         $mform->disabledIf('teamsubmissiongroupingid', 'teamsubmission', 'eq', 0);
+        $mform->setDefault('teamsubmissiongroupingid', 0);
 
         $mform->addElement('header', 'notifications', get_string('notifications', 'assign'));