Clean up shuffling cache leftovers from PR #9305

- 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.

beacon_node/beacon_chain/src/errors.rs

@@ -135,9 +135,6 @@ pub enum BeaconChainError {
         state_epoch: Epoch,
         shuffling_epoch: Epoch,
     },
-    MissingPtcForGloasShuffling {
-        shuffling_epoch: Epoch,
-    },
     SyncDutiesError(BeaconStateError),
     InconsistentForwardsIter {
         request_slot: Slot,