MDL-46212 question editing: extra answer fields notice.

It turns out that PHP does not interpret "$field[$key]" the way we were
expecting.

question/type/edit_question_form.php

@@ -613,7 +613,7 @@ protected function data_preprocessing_extra_answer_fields($question, $extraanswe
         foreach ($question->options->answers as $answer) {
             foreach ($extraanswerfields as $field) {
                 // See hack comment in {@link data_preprocessing_answers()}.
-                unset($this->_form->_defaultValues["$field[$key]"]);
+                unset($this->_form->_defaultValues["{$field}[{$key}]"]);
                 $extrafieldsdata[$field][$key] = $this->data_preprocessing_extra_answer_field($answer, $field);
             }
             $key++;