docs: Remove outdated subsystems article on guest users.

This page was no longer up to date, and these details should be
covered in more user-facing sections of our documentation.

docs/subsystems/guest-users.md

@@ -1,27 +0,0 @@
-# Guest users
-
-This page documents how guest users work in Zulip.  This documentation
-attempts to cover the current state first, and then the
-to-be-implemented restrictions.
-
-Guest users are like normal users in Zulip, except that they cannot do
-the following:
-
-* Join public streams without being added by another user.
-* Access message history on public streams.
-* Create streams (public or private)
-* Create or own bots users
-
-For many of these limitations, we haven't yet hidden the relevant
-section(s) of the UI, and the prototype guest user experience will
-feel buggy until we hide those features.
-
-Limitations to be implemented:
-* Manage streams (e.g. Add other users to a stream they are subscribed
-  to)
-* See streams they are not subscribed to.
-* Interact with user groups
-* Send private messages to users not in a stream with them.
-
-(And more, this is an initial high-level TODO list).
-

docs/subsystems/index.rst

@@ -32,7 +32,6 @@ Subsystems Documentation
    logging
    typing-indicators
    users
-   guest-users
    django-upgrades
    release-checklist
    api-release-checklist