Remove broken GitHub Pages report

[pquentin]

The last successful update was in 2022, and the report looked like this:

• https://elastic.github.io/elasticsearch-specification/report.html
• https://elastic.github.io/elasticsearch-specification/namespace-dependencies-expanded.html
• https://elastic.github.io/elasticsearch-specification/namespace-dependencies-compact.html

I don't think there's enough value to keep maintaining it, and having the CI failing makes it look like our CI is broken when it's not.