chore(gamedata): drop keys orphaned by the once-per-field rewrite

Remove three gamedata entries no code resolves anymore:
- CFlattenedSerializer::EncodeField — RE-exploration leftover, never hooked
- CNetworkGameServer::SendClientMessages — its own note said the result was
  discarded; the rewrite stopped resolving it entirely
- CNetworkSerialization::SerializerSingleton — only used by the removed
  force-resend (Option D) slot-14 path

Refresh the stale header note. Keep CFlattenedSerializer::Encode as the
documented DO-NOT-HOOK guardrail (recursive 100KB frame; anchors the Windows
EncodeEntity derivation). Every other key is resolved at runtime.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>