Impl oneshot_broadcast for committee promises (#3595)

## Issue Addressed

NA

## Proposed Changes

Fixes an issue introduced in #3574 where I erroneously assumed that a `crossbeam_channel` multiple receiver queue was a *broadcast* queue. This is incorrect, each message will be received by *only one* receiver. The effect of this mistake is these logs:

```
Sep 20 06:56:17.001 INFO Synced                                  slot: 4736079, block: 0xaa8a…180d, epoch: 148002, finalized_epoch: 148000, finalized_root: 0x2775…47f2, exec_hash: 0x2ca5…ffde (verified), peers: 6, service: slot_notifier
Sep 20 06:56:23.237 ERRO Unable to validate attestation          error: CommitteeCacheWait(RecvError), peer_id: 16Uiu2HAm2Jnnj8868tb7hCta1rmkXUf5YjqUH1YPj35DCwNyeEzs, type: "aggregated", slot: Slot(4736047), beacon_block_root: 0x88d318534b1010e0ebd79aed60b6b6da1d70357d72b271c01adf55c2b46206c1
```

## Additional Info

NA

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -4609,13 +4609,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
             metrics::stop_timer(committee_building_timer);
 
-            if let Err(e) = sender.send(committee_cache.clone()) {
-                debug!(
-                    self.log,
-                    "Did not fulfil committee promise";
-                    "error" => %e
-                )
-            }
+            sender.send(committee_cache.clone());
 
             map_fn(&committee_cache, shuffling_decision_block)
         }