chore(gamedata): drop unused CFlattenedSerializer::Encode key

Not resolved at runtime (capture is at EncodeEntity); was kept only as a
DO-NOT-HOOK reference. The rationale lives in EncodeEntity's comment +
docs/ENCODE_CALLGRAPH_RE.md, so the dead key is removed. 16 keys remain.

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