user docs: Remove doc for digest emails.

This hasn't been on in production for months.

templates/zerver/help/configure-email-digest-notifications.md

@@ -1,10 +0,0 @@
-# Configure email digest notifications
-
-Follow the following steps to configure your settings to receive a weekly digest email
-with a summary of the activity on your Zulip organization, such as popular
-conversations and new users, while you were away.
-
-{settings_tab|notifications}
-
-2. Select the **Send digest emails when I'm away** option under the
-**Other notification settings** section.

templates/zerver/help/include/sidebar_index.md

@@ -83,7 +83,6 @@
 * [PMs, mentions, and alerts](/help/pm-mention-alert-notifications)
 * [Mute or unmute a stream](/help/mute-a-stream)
 * [Mute or unmute a topic](/help/mute-a-topic)
-* [Digest emails](/help/configure-email-digest-notifications)
 * [Alert words](/help/add-an-alert-word)
 * [Notify everyone on a stream](/help/notify-everyone-on-a-stream)
 * [Troubleshoot desktop notifications](/help/troubleshooting-desktop-notifications)