Fix failing DS scan by removing fixtures dir

What does this MR do?

Remove fixture directory so that it is not scanned by gemnasium.

See successful gemnasium-dependency_scanning job: https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/jobs/910203841

What are the relevant issue numbers?

gitlab-org/gitlab#292693 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
  • See https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/jobs/910203841
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer