Tweak Mattermost notification docs not to specify a channel

The Bonus Notification Strategies section of the admin guide instructs the user to specify a channel in the Mattermost configuration properties. It appears that a bug in Mattermost stops this working, but removing the channel from the configuration properties lets it work again. The upshot is that, for now, notifications can target only whichever channel or private group the incoming webhook is tied to on the Mattermost side.