Stop fetching gitaly feature flags for performance concern

What does this MR do and why?

This MR stops fetching gitaly feature flags for performance concern, especially Redis.

We investigated the performance issue for GET /api/internal/kubernetes/agent_info. A possible cause is Redis, and fetching feature flags (which will be cached to Redis) could impact Redis. So, we will stop fetching them, and check the latency after shipping this MR.

For more context, here is the related discussion.

Part of #362075 (closed)

How to set up and validate locally

1. $ gdk start && gdk stop gitlab-k8s-agent
2. Run gitlab-agent locally. Follow steps here - running-kas-and-agentk-locally.
3. Gitlab Agent will request GET /agent_info when it starts.
4. Check no error happens at KAS side. KAS will generate error logs if it fails to get agent info, for example: {"level":"error","time":"2023-12-13T17:38:13.871+0900","msg":"AgentInfo()","grpc_service":"gitlab.agent.reverse_tunnel.rpc.ReverseTunnel","grpc_method":"Connect","error":"HTTP status code: 500 for path /api/v4/internal/kubernetes/agent_info"}

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