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>

slasher/src/slasher.rs

@@ -244,7 +244,7 @@ impl<E: EthSpec> Slasher<E> {
             let slashing_status = self.db.check_and_update_attester_record(
                 txn,
                 validator_index,
-                &attestation,
+                attestation,
                 attester_record,
             )?;