chore: remove support for custom Terraform checks

[nikpivkin]

Description

Part of https://github.com/aquasecurity/trivy/issues/7718

Related PRs

• [ ] https://github.com/aquasecurity/trivy/pull/7867

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Checklist

• [ ] I've read the guidelines for contributing to this repository.
• [ ] I've followed the conventions in the PR title.
• [ ] I've added tests that prove my fix is effective or that my feature works.
• [ ] I've updated the documentation with the relevant information (if needed).
• [ ] I've added usage information (if the PR introduces new options)
• [ ] I've included a "before" and "after" example to the description (if the PR is a user interface change).