Remove the additional_repo_storage_by_namespace FF

What does this MR do?

This MR removes the feature flag additional_repo_storage_by_namespace, both the references to the flag and the definition yml file.

Note: References to the additional_repo_storage_by_namespace? method on Namespace still remain as those are still relevant. There are some places on the front end that reference a flag but those are referencing this method and not the feature flag specifically.

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 #255166