[Feature flag] Cleanup mr_vulnerability_code_flow

What does this MR do and why?

Removing completely because this feature flag is not necessary anymore. This feature was deploy in 17.7 milestone.

[Feature flag] Cleanup mr_vulnerability_code_flow

Changelog: other

References

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MR acceptance checklist

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Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Create an empty project
3. Go to your locally project and edit with web IDE
4. Create a new file called: gl-sast-report.json and copy the content from here: gl-sast-report.json
5. Create a new file .gitlab-ci.yml and pasted this content: gitlab-ci.yml
6. Create a new folder called app and a new file inside called app.py and pasted this content: app.py
7. Create a new MR
8. Go into 'Changes' in your MR and you will see the detected vulnerability using shapes next to the specific rows.
9. Verify that all content in 'Code flow' tab behaves as expected

Numbered steps to set up and validate the change are strongly suggested.

Related to #504344 (closed)