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/http_api/src/attester_duties.rs

@@ -63,7 +63,7 @@ fn cached_attestation_duties<T: BeaconChainTypes>(
         .map_err(warp_utils::reject::beacon_chain_error)?;
 
     let (duties, dependent_root) = chain
-        .validator_attestation_duties(&request_indices, request_epoch, head.block_root)
+        .validator_attestation_duties(request_indices, request_epoch, head.block_root)
         .map_err(warp_utils::reject::beacon_chain_error)?;
 
     convert_to_api_response(duties, request_indices, dependent_root, chain)
@@ -104,7 +104,7 @@ fn compute_historic_attester_duties<T: BeaconChainTypes>(
         )?;
         state
     } else {
-        StateId::slot(request_epoch.start_slot(T::EthSpec::slots_per_epoch())).state(&chain)?
+        StateId::slot(request_epoch.start_slot(T::EthSpec::slots_per_epoch())).state(chain)?
     };
 
     // Sanity-check the state lookup.