Remove api_billable_member_list feature flag

What does this MR do?

The api_billable_member_list feature flag is stable enough, so it is safe to remove.

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.

Related to #254947 (closed)