docs: Remove outdated section on sending push notifications directly.

With recent work on E2EE, instructions on sending push notifications
directly (without using the service) have become outdated. As questions
about how to do this generally come from users whose goal is
to work around the business model that supports Zulip's development,
we are dropping these instructions rather than updating them. E2EE
should eliminate other motivations for using this documentation.

docs/production/mobile-push-notifications.md

@@ -441,66 +441,3 @@ the `PUSH_NOTIFICATION_BOUNCER_URL = 'https://push.zulipchat.com'` line in your
 `/etc/zulip/settings.py` file (i.e., add `# ` at the start of the line), and
 [restart your Zulip server](settings.md#changing-server-settings). This approach makes it
 easy to start using the service again by uncommenting the same line.
-
-## Sending push notifications directly from your server
-
-This section documents an alternative way to send push notifications
-that does not involve using the Mobile Push Notification Service at
-the cost of needing to compile and distribute modified versions of the
-Zulip mobile apps.
-
-We don't recommend this path -- patching and shipping a production
-mobile app can take dozens of hours to set up even for an experienced
-developer, and even more time to maintain. And it doesn't provide
-material privacy benefits -- your organization's push notification
-data would still go through Apple/Google's servers, just not Kandra
-Labs'. But in the interest of transparency, we document in this
-section roughly what's involved in doing so.
-
-As [discussed above](#why-a-push-notification-service-is-necessary),
-it is impossible for a single app in the Google or Apple
-store to receive push notifications from multiple, mutually
-untrusted, servers. The Mobile Push Notification Service is one of
-the possible solutions to this problem.
-
-The other possible solution is for an individual Zulip server's administrators
-to build and distribute their own copy of the Zulip mobile apps, hardcoding a
-key that they possess. This solution is possible with Zulip, but it requires the
-server administrators to publish their own copies of the Zulip mobile apps.
-There's nothing the Zulip team can do to eliminate this onerous requirement.
-
-The main work is thus distributing your own copies of the Zulip mobile apps
-configured to use APNS/FCM keys that you generate. This is not for
-the faint of heart! If you haven't done this before, be warned that
-one can easily spend hundreds of dollars (on things like a DUNS number
-registration) and a week struggling through the hoops Apple requires
-to build and distribute an app through the Apple app store, even if
-you're making no code modifications to an app already present in the
-store (as would be the case here). The Zulip mobile app also gets
-frequent updates that you will have to either forgo or republish to
-the app stores yourself.
-
-If you've done that work, the Zulip server configuration for sending
-push notifications through the new app is quite straightforward:
-
-- Create an
-  [FCM push notifications](https://firebase.google.com/docs/cloud-messaging)
-  key in the Google Developer console and set `android_gcm_api_key` in
-  `/etc/zulip/zulip-secrets.conf` to that key.
-
-- In Apple's developer console, register a [token][apple-doc-token] or
-  [certificate][apple-doc-cert] for sending push notifications.
-  Then in `/etc/zulip/settings.py`, set `APNS_SANDBOX=False`, and:
-
-  - If using APNs [certificate-based authentication][apple-doc-cert],
-    set `APNS_CERT_FILE` to the path of your APNs certificate file.
-
-  - If using APNs [token-based authentication][apple-doc-token],
-    set `APNS_TOKEN_KEY_FILE` to the path of your APNs token key file,
-    `APNS_TOKEN_KEY_ID` to the corresponding 10-character key ID, and
-    `APNS_TEAM_ID` to your 10-character Apple team ID.
-
-- Restart the Zulip server.
-
-[apple-doc-cert]: https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/establishing_a_certificate-based_connection_to_apns
-[apple-doc-token]: https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/establishing_a_token-based_connection_to_apns