Rust 1.54.0 lints (#2483)

## Issue Addressed

N/A

## Proposed Changes

- Removing a bunch of unnecessary references
- Updated `Error::VariantError` to `Error::Variant`
- There were additional enum variant lints that I ignored, because I thought our variant names were fine
- removed `MonitoredValidator`'s `pubkey` field, because I couldn't find it used anywhere. It looks like we just use the string version of the pubkey (the `id` field) if there is no index

## Additional Info



Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/beacon_chain/src/sync_committee_verification.rs

@@ -573,7 +573,7 @@ pub fn verify_signed_aggregate_signatures<T: BeaconChainTypes>(
     let signature_sets = vec![
         signed_sync_aggregate_selection_proof_signature_set(
             |validator_index| pubkey_cache.get(validator_index).map(Cow::Borrowed),
-            &signed_aggregate,
+            signed_aggregate,
             &fork,
             chain.genesis_validators_root,
             &chain.spec,
@@ -581,7 +581,7 @@ pub fn verify_signed_aggregate_signatures<T: BeaconChainTypes>(
         .map_err(BeaconChainError::SignatureSetError)?,
         signed_sync_aggregate_signature_set(
             |validator_index| pubkey_cache.get(validator_index).map(Cow::Borrowed),
-            &signed_aggregate,
+            signed_aggregate,
             &fork,
             chain.genesis_validators_root,
             &chain.spec,