Remove unused deploy script

Once upon a time we included a deploy script when creating a site from the tech docs template if you said you were going to be deploying to PaaS [1].

We also now deploy to GitHub Pages instead of PaaS (And, as far as I can tell we’ve never used it – the deploy has always been handled by GitHub Actions or Concourse.)

So delete the unused deploy script.

[1]: https://github.com/alphagov/tech-docs-template/blob/main/optional/script/deploy