refactor(template): remove unused test

[knqyf263]

Description

We moved tests to sarif_test.go and forgot to remove this test function.
https://github.com/aquasecurity/trivy/blob/main/pkg/report/sarif_test.go

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).