Remove vue_admin_users feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR removes the vue_admin_users feature flag introduced in !48922 (merged).

The feature flag defaults to on, and the HAML views are no longer being rendered and can be removed in this MR.

Testing & Setup

1. You need a GitLab Ultimate license for all the features.
2. View the Admin Users page at [GDK_HOST]/admin/users.

Screenshots (strongly suggested)

No visual change.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry. In commit message
  • I have not included a changelog entry because _____.
• [-] Documentation (if required) - No visual change and feature flag not mentioned.
• 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 #290737 (closed)