puppet: Remove zulip_deliver_scheduled_* from zulip-workers:*.

Staging and other hosts that are `zulip::app_frontend_base` but not
`zulip::app_frontend_once` do not have a
/etc/supervisor/conf.d/zulip/zulip-once.conf and as such do not have
`zulip_deliver_scheduled_emails` or `zulip_deliver_scheduled_messages`
and thus supervisor will fail to reload.

Making the contents of `zulip-workers` contingent on if the server is
_also_ a `-once` server is complicated, and would involve using Concat
fragments, which severely limit readability.

Instead, expel those two from `zulip-workers`; this is somewhat
reasonable, since they are use an entirely different codepath from
zulip_events_*, using the database rather than RabbitMQ for their
queuing.

scripts/stop-server

@@ -40,6 +40,13 @@ if has_application_server():
     services.append("zulip-django")
     services.extend(["zulip-tornado", "zulip-tornado:*"])
     services.append("zulip-workers:*")
+    if has_application_server(once=True):
+        services.extend(
+            [
+                "zulip_deliver_scheduled_emails",
+                "zulip_deliver_scheduled_messages",
+            ]
+        )
 
 subprocess.check_call(["supervisorctl", "stop", *services])