Remove the Docker in Docker feature configuration

[StevenMaude]

Fixes #127.

This PR removes the Docker in Docker dev container feature configuration to simplify the devcontainer.json.

Much of the configuration simply restates the feature defaults.

This was all created by Visual Studio Code originally; see ef7549f48bee0debf63e97a87a93feb8e5ea1e0a.

See the commit messages for more detail.

[StevenMaude]

The one thing from this I learned is that we're using Moby and not Docker CE.

I'm not sure that matters too much as the two are closely related, but we could explicitly choose Docker if we like.

[StevenMaude]

My suggested minimal acceptance criteria for such a change are that the following work as expected:

• [x] ehrQL autocompletion
• [x] opensafely run run_all
• [x] can connect to R Studio server