Fix namespace billing page incorrectly accessing billable members

What does this MR do?

Fix customers-gitlab-com#2327 (closed)

The namespace's billing page is trying to access billable members API endpoint, which would only work for groups.

This MR tries to stop namespace billing page from making this request.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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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

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• 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
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