Group Registry Browser: remove feature flag

What does this MR do?

Since the two feature flags: group_container_registry_browser and vue_container_registry_explorer are both enabled for everyone on gitlab.com since more than a week this MR removes them and removes all the old code that is not reachable anymore.

Screenshots

No visual changes

Where to check this:

• project level: https://gitlab.com/gitlab-org/gitlab/container_registry
• group level: https://gitlab.com/groups/gitlab-org/-/container_registries

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 #23315 (closed)