Clean up blob_repository_vue_header_app flag

What does this MR do and why?

This MR includes commits cherry-picked from Clean up blob_repository_vue_header_app flag that we had to revert due to found bug. The element that caused the bug was removed entirely with the refactor of blob controls. Now that Clean up blob_overflow_menu feature flag has been cleaned up, we can default to the new architecture.

References

• #513582 (closed)
• flag was turned on in !176182 (merged)
• revert MR after we discovered problems

Screenshots or screen recordings

Before	After
on master with ff turned on as default Screen_Recording_2025-06-05_at_18.24.11	this branch with feature flag removed Screen_Recording_2025-06-05_at_18.22.55

How to set up and validate locally

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

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #540934 (closed)