featureflag: Remove concurrency_queue_enforce_max feature flag

Now that the concurrency_queue_enforce_max has been running in staging and production without issue, we can remove it.

https://dashboards.gitlab.net/d/CP23bQfnz/gitaly-request-limiting?orgId=1 has a "Dropped Requests - Max Queue Size" graph that shows no data, which is because there have no requests dropped since we set the limit very high.