bug: remove unused files. (#15738)

The files were:
 - created 6y ago without major updates.
 - using Ubuntu 16.04.
 - using S3 bucket that does not exist anymore (and creates security
   vulnerability).
 - not used in the repository.

After change - there are no references to the deleted files:
```bash
❯ ag "Dockerfile.dev"
❯ ag "setup-h2o-dev.sh"
  ~/Devel/projects/h2o/repos/h2o2 on   mm/master/gh_15737 *43 +2 ?19 ❯
```

This change fixes #15737
and related security vulnerability https://huntr.mlsecops.com/bounties/6a69952f-a1ba-4dee-9d8c-e87f52508b58/