Remove the gitlab_sli_new_counters feature flag

What does this MR do and why?

The feature flag has been enabled since 2022-07-06 https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/12403

In gitlab-com/runbooks!4801 (merged) we stopped using the old metrics for SLIs and started using the new ones.

The last step in gitlab-com/gl-infra/scalability#1760 (closed)

How to set up and validate locally

Check out this branch and restart the GDK, click around a little. When querying metrics you should only see metrics separated by underscores:

→ curl -s http://localhost:3000/-/metrics | grep gitlab_sli                                                                                                                                               
# HELP gitlab_sli_rails_request_apdex_success_total Multiprocess metric
# TYPE gitlab_sli_rails_request_apdex_success_total counter
gitlab_sli_rails_request_apdex_success_total{endpoint_id="SessionsController#new",feature_category="authentication_and_authorization",request_urgency="low"} 1
gitlab_sli_rails_request_apdex_success_total{endpoint_id="unknown",feature_category="unknown",request_urgency="default"} 17
# HELP gitlab_sli_rails_request_apdex_total Multiprocess metric
# TYPE gitlab_sli_rails_request_apdex_total counter
gitlab_sli_rails_request_apdex_total{endpoint_id="RootController#index",feature_category="projects",request_urgency="low"} 2
gitlab_sli_rails_request_apdex_total{endpoint_id="SessionsController#new",feature_category="authentication_and_authorization",request_urgency="low"} 1
gitlab_sli_rails_request_apdex_total{endpoint_id="unknown",feature_category="unknown",request_urgency="default"} 17

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.