Remove cascade_package_forwarding_settings feature flag

What does this MR do and why?

This removes the cascade_package_forwarding_settings feature flag that was introduced in !99285 (merged). It has been enabled on GitLab.com since 2022-10-14.

Rollout issue: #375761 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #375761 (closed)