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>

testing/ef_tests/src/cases/epoch_processing.rs

@@ -129,8 +129,8 @@ impl<E: EthSpec> EpochTransition<E> for Slashings {
     fn run(state: &mut BeaconState<E>, spec: &ChainSpec) -> Result<(), EpochProcessingError> {
         match state {
             BeaconState::Base(_) => {
-                let mut validator_statuses = base::ValidatorStatuses::new(&state, spec)?;
-                validator_statuses.process_attestations(&state)?;
+                let mut validator_statuses = base::ValidatorStatuses::new(state, spec)?;
+                validator_statuses.process_attestations(state)?;
                 process_slashings(
                     state,
                     validator_statuses.total_balances.current_epoch(),