Retire Node Eligibility Service (NES) [Closing Down]

### Value Prop
GitHub Enterprise Server is removing an unused service, Node Eligibility Service (NES), to keep the platform lean and well-maintained. This cleanup reduces unnecessary complexity in your self-hosted environment without affecting any customer-facing functionality.

### Expected Outcome
Retiring this service eliminates dead code and maintenance burden that provided no customer value.

The roadmap issue names Node Eligibility Service (NES) but provides no repository files, tests, or entry point. Start by locating NES in the GitHub Enterprise Server codebase and confirming its ownership and dependencies; done means the unused service is retired without customer-facing impact.

索引モデルが issue の本文から書いたものです。