disable fdroidclassic repo by default
Now that the app is in the main f-droid repository, (with the same signature) the f-droid classic repo can be made optional as the app will get updates via f-droid main.
Maybe it can be enabled for beta releases (which currently are not in f-droid main) but maybe we don't even need that as people who use beta releases should be capable of enabling the repo in settings.