Removing unneeded key for free search service

[pamelafox]

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Purpose

I had previously added support for using a key for search service because I thought the free tier didn't support managed identity. As it turns out, it does support authenticating with managed identity- it just doesn't support having its own identity (i.e. outbound identity). That's necessary for things like integrated vectorization for it to auth to services, but we already don't expect integrated vectorization to work well on free tier.

So this PR makes free tier the same in terms of using keyless auth.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error? If you're not sure, try it out on an old environment.

[ ] Yes
[X] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by this tutorial which includes deployment, settings and usage instructions. If text or screenshot need to change in the tutorial, check the box below and notify the tutorial author. A Microsoft employee can do this for you if you're an external contributor.

[ ] Yes
[X] No

Type of change

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

Code quality checklist

See CONTRIBUTING.md for more details.

• [x] The current tests all pass (python -m pytest).
• [ ] I added tests that prove my fix is effective or that my feature works
• [x] I ran python -m pytest --cov to verify 100% coverage of added lines
• [ ] I ran python -m mypy to check for type errors
• [x] I either used the pre-commit hooks or ran ruff and black manually on my code.

[pamelafox]

And yes, some of this code may come back if I figure out keyvault storage for Entra secrets, but it may be in a different form, so I don't want to leave it in just in case - I'll just bring back what's needed.