Clean up `blob_repository_vue_header_app` flag

What does this MR do and why?

Clean up blob_repository_vue_header_app flag and removed unused files

References

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• #513582 (closed)
• flag was turned on in !176182 (merged)

MR acceptance checklist

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How to set up and validate locally

See #512957 (closed) for breakdown of features to test

Related to #513582 (closed)