Port to using azd feature

[pamelafox]

Purpose

There's now an azd feature for dev containers, which will automatically download the appropriate azd for your architecture (arm/amd) so platform emulation is no longer needed.
This PR updates the dev container to use that feature and also removes some other unnecessary bits.

Does this introduce a breaking change?

[ ] Yes [X] No 

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix [X] Feature [ ] Code style update (formatting, local variables) [ ] Refactoring (no functional changes, no api changes) [ ] Documentation content changes [ ] Other... Please describe: 

How to Test

• Open in Dev Container in VS Code, confirm azd command works, server can be run
• Open in GitHub Codespaces, ditto

[pamelafox]

@bobtabor-msft Please review