Slack integration is broken due to deprecated/broken Slacker

**Problem**
I tried setting up `ntfy` for slack notifications, but looks like Slacker is broken, so it's no longer able to make requests via the slack api.

I tried to make a PR with slacker to get [this fix in](https://github.com/ismith/slacker/commit/8ff928c59ec77b047b7270d96936e3df7e2bc4fb), it looks like the project got archived and is no longer maintained.

**Recommendation**
I think it's best to remove this slack option and recommend users to set-up slack webhooks instead.