doc: remove OpenJSF Slack nodejs from support doc
The OpenJSF Slack nodejs channel has a description that reads:
🚨 For Node.js application development questions, see:
- Node Slackers: https://www.nodeslackers.com/
- Node.js help repo: https://github.com/nodejs/help
Let's not send people with support questions there just to get ignored or redirected elsewhere.