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/operation_pool/src/lib.rs

@@ -383,7 +383,7 @@ impl<T: EthSpec> OperationPool<T> {
 
         let relevant_attester_slashings = reader.iter().flat_map(|(slashing, fork)| {
             if *fork == state.fork().previous_version || *fork == state.fork().current_version {
-                AttesterSlashingMaxCover::new(&slashing, &to_be_slashed, state)
+                AttesterSlashingMaxCover::new(slashing, &to_be_slashed, state)
             } else {
                 None
             }

beacon_node/operation_pool/src/max_cover.rs

@@ -121,7 +121,7 @@ mod test {
         }
 
         fn covering_set(&self) -> &Self {
-            &self
+            self
         }
 
         fn update_covering_set(&mut self, _: &Self, other: &Self) {