Remove the Docker-in-Docker configuration

[StevenMaude]

We dropped this for the research-template.

See opensafely/research-template#129.

[StevenMaude]

Tried this out, and #6 is resolved with this change.

[StevenMaude]

[StevenMaude]

For the record, I'll note that this change only fixed the problem incidentally.

Having looked more — and for future reference — this is what I think happened:

• Docker Compose has a v2.29.2 tag, but doesn't currently have a v2.29.2 release.
  • This is possibly because their releases via CI are set to draft by default, and then presumably get manually published(?).
  • Failing to download v2.29.2 is what was failing in the original issue
• We specified v2 as dockerDashComposeVersion in devcontainer.json.
  • The dev container feature shell script finds the latest v2 GitHub release version via looking at tags, and then tries to download that tag's release.
  • This is what was causing the failures; a v2.29.2 release didn't exist.
• If the dockerDashComposeVersion isn't specified in the Docker-in-Docker configuration, then the option used in the dev container feature shell script is latest.
  • latest also tries to find the latest version, but…
  • when using latest, there's also a fallback if it fails, which tries to fetch the previous version; this does work because v2.29.1 does exist

That said, it's still better to keep the dev container configuration as close to the research-template as possible (see also #7), and it reduces the amount of configuration here too.

I'd guess we're not really using docker-compose in this repository, so the version set shouldn't matter too much.