UPDATE

You can update you issue. After you have removed the content you don't want visible you can now delete the revisions from the issue history.

Note: Any attachment might still online if the URL is known (search engine cache, old browser tab, ...). If you don't Gthub to have the content, contact their support as suggested in Ionică Bizău's answer.

Editing the issue and deleting the revision from history is curently the fastest way to remove content from public visibility.