It's not something hard coded in a template that you can just edit and remove.
The content title is fetched via the function getContentTitle() in the Content Entity.
PHP:
public function getContentTitle(string $context = '')
{
return \XF::phrase('thread_x', ['title' => $this->title]);
}
Anyway, XF developers listened to the feedback on this and XF 2.4 includes a solution that the content type prefix only will be displayed if the widget contains mixed content