Can't make subscriptions default feed type in gest mode

[micahmo]

Pull Request Description

This PR removes the option to set "Subscribed" as the default feed type for guest mode, since that is not a valid option. I also added a migration in case anyone had that set before.

Note that, although we did allow "Subscribed" as a default option, it didn't seem to actually cause a problem. Having "Subscribed" selected from a user account also didn't cause a problem with switching to an anonymous instance.

Issue Being Fixed

Issue Number: #1487

Screenshots / Recordings

This is just a screen recording to show that I couldn't reproduce any issue with the way things are now (aside from potential confusion).

qemu-system-x86_64_KR1Lf74moO.mp4

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM!