Revert function validation

[ttuegel]

Revert #2168.
Revert #2574.
Revert #2575.

The integration test in test/regression-c-semantics was silently failing on master due to these changes.

Because of the difficulty synchronizing the frontend with our prelude.kore, I will keep the changes to that file; the changes were backwards-compatible anyway.

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Review checklist

The author performs the actions on the checklist. The reviewer evaluates the work and checks the boxes as they are completed.

• Summary. Write a summary of the changes. Explain what you did to fix the issue, and why you did it. Present the changes in a logical order. Instead of writing a summary in the pull request, you may push a clean Git history.
• Documentation. Write documentation for new functions. Update documentation for functions that changed, or complete documentation where it is missing.
• Tests. Write unit tests for every change. Write the unit tests that were missing before the changes. Include any examples from the reported issue as integration tests.
• Clean up. The changes are already clean. Clean up anything near the changes that you noticed while working. This does not mean only spatially near the changes, but logically near: any code that interacts with the changes!