- Remove unused `BeaconChainError::MissingPtcForGloasShuffling` variant
(no producers remained after the earlier cleanup).
- Drop the `Result<(), BeaconChainError>` return type from
`ShufflingCache::insert_committee_cache`; both match arms are
infallible. Update callers in `beacon_chain.rs`, `state_advance_timer.rs`,
`shuffling_cache.rs` and the unit tests accordingly.
- Trim stale "Replace the committee if it's not present" comment in
`insert_committee_cache`; the Committee arm is now a no-op so only
the `Promise(_) | None` whimsy line remains.