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/validator_monitor.rs

@@ -126,8 +126,6 @@ type SummaryMap = HashMap<Epoch, EpochSummary>;
 struct MonitoredValidator {
     /// A human-readable identifier for the validator.
     pub id: String,
-    /// The validator voting pubkey.
-    pub pubkey: PublicKeyBytes,
     /// The validator index in the state.
     pub index: Option<u64>,
     /// A history of the validator over time.
@@ -140,7 +138,6 @@ impl MonitoredValidator {
             id: index
                 .map(|i| i.to_string())
                 .unwrap_or_else(|| pubkey.to_string()),
-            pubkey,
             index,
             summaries: <_>::default(),
         }