email: Drop support for SEND_MISSED_MESSAGE_EMAILS_AS_USER. (#27223)

Originally, this was how the notification emails worked, but that was changed
in 797a7ef97b, with this old behavior 
available as an option.

The footer and from address of emails that are sent when this
setting is set to True are confusing, especially when more people
are involved in a stream and since we have changed the way we send
emails, it should be removed. It’s also not widely used.

Fixes #26609.

zerver/lib/email_notifications.py

@@ -573,20 +573,6 @@ def do_send_missedmessage_events_reply_in_zulip(
     with override_language(user_profile.default_language):
         from_name: str = _("Zulip notifications")
     from_address = FromAddress.NOREPLY
-    if len(senders) == 1 and settings.SEND_MISSED_MESSAGE_EMAILS_AS_USER:
-        # If this setting is enabled, you can reply to the Zulip
-        # message notification emails directly back to the original sender.
-        # However, one must ensure the Zulip server is in the SPF
-        # record for the domain, or there will be spam/deliverability
-        # problems.
-        #
-        # Also, this setting is not really compatible with
-        # EMAIL_ADDRESS_VISIBILITY_ADMINS.
-        sender = senders[0]
-        from_name, from_address = (sender.full_name, sender.email)
-        context.update(
-            reply_to_zulip=False,
-        )
 
     email_dict = {
         "template_prefix": "zerver/emails/missed_message",