Remove rate limits feature flags

Summary

We rolled out rate limits for gitlab pages, so now we can remove feature flags guarding it.

SO we need to:

1. remove 4 relevant feature flags from https://gitlab.com/gitlab-org/gitlab-pages/-/blob/a8be317a96f2dfb4ab30e338daee2b51833c2322/internal/feature/feature.go#L12-L12
2. remove enforce flag from https://gitlab.com/gitlab-org/gitlab-pages/-/blob/master/internal/ratelimiter/ratelimiter.go#L48, and all tests using it
3. clean our production to not mention these feature flags: https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-com/-/blob/96a3bf149c1cd2847f29c3ed666c480e6b8d15b7/releases/gitlab/values/values.yaml.gotmpl#L449 , https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-com/-/blob/96a3bf149c1cd2847f29c3ed666c480e6b8d15b7/releases/gitlab/values/gprd.yaml.gotmpl#L97 , and other files, grep the repository for each of them
4. remove mentions of feature flags in https://docs.gitlab.com/ee/administration/pages/#rate-limits