Remove old index.html

[tedmiston]

Currently, there's an index.html (from 3 years ago) in the repo root /:

https://github.com/fly-apps/hello-static/blob/199d358e5f7e21bbf6c1eae8e74bcba568e6305e/index.html

But also a second (newer) index.html in /public:

https://github.com/fly-apps/hello-static/blob/199d358e5f7e21bbf6c1eae8e74bcba568e6305e/public/index.html

The latter appears to be the one that's deployed per the Dockerfile, so this PR removes the former, which appears to be a legacy artifact original index.html from before there were multiple files.