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>

consensus/state_processing/src/per_epoch_processing/base.rs

@@ -28,7 +28,7 @@ pub fn process_epoch<T: EthSpec>(
     //
     // E.g., attestation in the previous epoch, attested to the head, etc.
     let mut validator_statuses = ValidatorStatuses::new(state, spec)?;
-    validator_statuses.process_attestations(&state)?;
+    validator_statuses.process_attestations(state)?;
 
     // Justification and finalization.
     process_justification_and_finalization(state, &validator_statuses.total_balances, spec)?;

consensus/state_processing/src/per_epoch_processing/base/rewards_and_penalties.rs

@@ -60,7 +60,7 @@ pub fn process_rewards_and_penalties<T: EthSpec>(
         return Err(Error::ValidatorStatusesInconsistent);
     }
 
-    let deltas = get_attestation_deltas(state, &validator_statuses, spec)?;
+    let deltas = get_attestation_deltas(state, validator_statuses, spec)?;
 
     // Apply the deltas, erroring on overflow above but not on overflow below (saturating at 0
     // instead).