Don't send real broadcasts for preview training mode

We previously allowed MNOs to approve a broadcast themselves in training
mode and have it go out to their integration environment as per
https://github.com/alphagov/notifications-api/pull/3114

However, we want to remove this use case as it means we have to support
configuration for training mode services to do things like pick a
channel and send out alerts which we definteily don't want to do in
production.

By making this change, we reduce the chance of a single bug meaning an
alert will go out in prod that shouldn't.

Note, will also make it harder for development environment testing but I
think it is still worth it as https://www.pivotaltracker.com/story/show/177584959
will make it much harder in our code to allow some environments to send
alerts whilst in training mode.

app/broadcast_message/rest.py

@@ -214,7 +214,7 @@ def _create_broadcast_event(broadcast_message):
 
     dao_save_object(event)
 
-    if not broadcast_message.stubbed or current_app.config['NOTIFY_ENVIRONMENT'] in ['preview', 'development']:
+    if not broadcast_message.stubbed:
         send_broadcast_event.apply_async(
             kwargs={'broadcast_event_id': str(event.id)},
             queue=QueueNames.BROADCASTS