Remove Poseidon page

The [Poseidon
project](https://github.com/kubernetes-sigs/poseidon/commits/master)
is virtually dead:

- No PRs coming in since Dec 2019
- PRs related to code changes date back to April 2019.

By removing this page, we can save folks' effort on localizing the
content, i.e. #22265.

Author: tengqm