Update and remove CSS, and remove unused issue header code

What does this MR do?

The issue header was recently converted from Haml to Vue in !44440 (merged). Once stable, the feature flag and associated unused code was removed in several MRs so as not to have one huge MR.

This MR is the final one to remove unused code associated with this Haml to Vue migration. Some CSS was also identified as being unused or a duplicate of other CSS so this was refactored and removed to reduce the CSS in our bundle.

Some of the changes in this MR:

• Removed btn-nr, btn-spam, btn-reopen, btn-reopen-color as they have no associated CSS properties
• Replaced btn-remove and btn-red with btn-danger, then removed btn-remove and btn-red
• Replaced btn-register with btn-success, then removed btn-register

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• 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:

• 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)