Broken link / paragraph about Katacoda on the agent mode blog post

[robertobandini]

Hi, reading this post I noticed that the links to Katacoda are broken because it was closed.

So I guess the related links and paragraphs should be removed. If you want I can do a PR about this.

• https://prometheus.io/blog/2021/11/16/agent/
• https://github.com/prometheus/docs/blob/main/blog-posts/2021-11-16-agent.md