I just had a similar issue with the "(classic)" text being added to the quiz title automatically. It happened after a classic quiz was duplicated or replicated into a new quiz using the New Quizzes, with the same title.
Editing the old quiz did not work - "(classic)" is not included in the editable area for the quiz title. Deleting the new quiz completely did not work - "(classic)" still shows up in title of the old quiz even after the deletion of the new quiz.
What worked for me was to slightly change the quiz title (e.g. from "Quiz 1" to "Module 1 Quiz"), and that made the system remove the "(classic)" text from the quiz title automatically.