Remove logging to slack

[aelkiss]

With logging via Loki and alerting via the pushgateway, there isn't as much need for this.

I waffled a bit as to whether to remove MultiLogger and SlackWriter entirely, but I think it makes sense to do so for a couple reasons:

1. there's no current need for it
2. if we have a need for this in the future we can use loki and alertmanager to selectively alert based on certain things being logged
3. we can revert later if need be if we have a scenario where we want to send things to slack outside of normal logging/alerting