MDL-27799 question bank: remove last traces of qtype constants.

Using consants to refer to plugin names is crazy. The whole point of
plugins is that you can install more, hence the list of constants will
never be complete.

question/format/learnwise/format.php

@@ -125,7 +125,7 @@ class qformat_learnwise extends qformat_default {
         }
 
         $question = $this->defaultquestion();
-        $question->qtype = MULTICHOICE;
+        $question->qtype = 'multichoice';
         $question->name = substr($questiontext, 0, 30);
         if (strlen($questiontext) > 30) {
             $question->name .= '...';