Use the Vue comparison selector on the repo compare show page

What does this MR do?

In !52567 (merged) we introduced a Vue control for building a comparison between two branches. As part of #14615 (closed) , we're added a "project" select box to that vue control.

We still render the old-style control on the compare "show" page (flow for this goes index -> create -> show). Without that project select box, it won't be clear which comparison we're showing. The simplest fix, I feel, is to just re-use the control on both pages - which lets us remove the haml-based one.

Screenshots (strongly suggested)

Show page (after, with compare_repo_dropdown FF enabled)

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 #300853 (closed)