[Feature flag] Cleanup multiple_billing_account_members_existing_users FF

Summary

This issue is to cleanup the multiple_billing_account_members_existing_users feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.

This feature flag allows customers to add additional customers as billing account managers for that account.

Owners

• Team: Subscription Management
• Most appropriate Slack channel to reach out to: #g_subscription_management
• Best individual to reach out to: @aluthra2 @dzubova
• PM: @tgolubeva

Stakeholders

groupsubscription management

Expectations

What might happen if this goes wrong?

Cleanup

After the feature has been deemed stable, the clean up should be done as soon as possible to permanently enable the feature and reduce complexity in the codebase.

• Create a merge request to remove <feature-flag-name> feature flag
  • Remove all references to the feature flag from the codebase
  • Remove references under /qa directory if applicable, and notify SETs
• Ensure that the cleanup MR has been deployed to staging, staging-ref and production
• Disable the feature flag via the CustomersDot dashboard
• Ensure production environment is still working as expected
• Remove the Unleash feature flag from the CustomersDot dashboard
• Close this rollout issue.