Remove references to the upstream database in the API

Fixes

This is a follow up to this comment on the API ECS migration effort: https://github.com/WordPress/openverse-infrastructure/pull/323#discussion_r1055052813

For those without access, @sarayourfriend pointed out that the UPSTREAM_* variables could be removed from the API, since there are ostensibly no references to it in the API code.

Description

This PR removes the upstream database configuration and the related UPSTREAM_* variables from the API. I've run both the unit tests and the integration tests locally, and also navigated through the API a bit. Everything looks fine!

Testing Instructions

Build and play around with the API to make sure everything still works.

Checklist

[x] My pull request has a descriptive title (not a vague title like Update index.md).
[x] My pull request targets the default branch of the repository (main) or a parent feature branch.
[x] My commit messages follow [best practices][best_practices].
[x] My code follows the established code style of the repository.
[ ] I added or updated tests for the changes I made (if applicable).
[ ] I added or updated documentation (if applicable).
[x] I tried running the project locally and verified that there are no visible errors.

[best_practices]: https://git-scm.com/book/en/v2/Distributed-Git-Contributing-to-a-Project#_commit_guidelines

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WordPress / openverse-api