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.
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