Cleanup the `packages_dependency_proxy_containers_scope_check` feature flag

Background

Parent issue: #426887

We planned to introduce scope checks in !136655 (merged), behind the feature flag packages_dependency_proxy_containers_scope_check. The plan was to pass scopes to the controller, so that we can have controller or service code that relies on the policies for scope checking. We eventually realized that this approach will not work for all tokens, due to how our authentication system handles tokens.

Instead we plan to implement scope checking during authentication, similar to how Container registry is doing it. This will be implemented through the issues below:

• Implement scope logging during authentication - !181756 (merged)
• Implement scope checking during authentication, behind a feature flag - !182559 (merged)
• Create migration documentation
• Test impact on existing workflows
• Rollout the feature flag
• Cleanup the packages_dependency_proxy_containers_scope_check feature flag <-- we are here

After everything has been done, we clean up the code related to packages_dependency_proxy_containers_scope_check in this issue.