Use default devcontainer

Remove the obsolete bespoke devcontainer config, and instead use the
[default devcontainer][1].

We will add any useful bespoke devcontainer config in a future commit,
but the default devcontainer is fine for now.

[1]: https://docs.github.com/en/codespaces/setting-up-your-project-for-codespaces/adding-a-dev-container-configuration/introduction-to-dev-containers#using-the-default-dev-container-configuration

Author: johnboyes