Drop slack integration

[kentsanggds]

As any issues on publish do not directly impact users we probably don't need to be alerted to any issues with the workers on slack.

We do probably need something around an alert when an update doesn't happen within 4 hours or so as this would indicate an issue with the stack.

The slack updates are currently creating noise whenever there is an upgrade of kubernetes or when the shared-redis pod gets recycled as there will be a some downtime.