test: Remove tests wrt foreign key

[Mariatta]

There is no restriction about this in the backend.
Removing the tests since they're not needed.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #610 🦕
Fixes #611 🦕

Closes internal issue #137866686