Remove `vue_issue_header` feature flag code
What does this MR do?
This MR removes the vue_issue_header feature flag code. This feature flag was introduced in !44440 (merged) to toggle between the old Haml template issue header and the new simplified Vue issue header, and the feature flag was defaulted to on in !47690 (merged).
In details_header.html.haml the feature flag is removed, and in this file the now unused else block is also removed along with the unused linked Haml templates. One additional change is that close_reopen_button.html.haml was deleted and its contents were moved into mr_title.html.haml since it's only used there now, and the code was simplified there.
Screenshots (strongly suggested)
No visual changes as the feature flag is defaulted to on. Screenshots are attached below to confirm.
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Does this MR meet the acceptance criteria?
Conformity
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Changelog entry -
Documentation (if required) -
Code review guidelines -
Merge request performance guidelines -
Style guides -
Database guides -
Separation of EE specific content
Availability and Testing
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Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process. -
Tested in all supported browsers -
Informed Infrastructure department of a default or new setting change, if applicable per definition of done
Security
If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:
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Label as security and @ mention @gitlab-com/gl-security/appsec -
The MR includes necessary changes to maintain consistency between UI, API, email, or other methods -
Security reports checked/validated by a reviewer from the AppSec team
Related to #276444 (closed)