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/builder.rs

@@ -666,7 +666,7 @@ fn genesis_block<T: EthSpec>(
     genesis_state: &mut BeaconState<T>,
     spec: &ChainSpec,
 ) -> Result<SignedBeaconBlock<T>, String> {
-    let mut genesis_block = BeaconBlock::empty(&spec);
+    let mut genesis_block = BeaconBlock::empty(spec);
     *genesis_block.state_root_mut() = genesis_state
         .update_tree_hash_cache()
         .map_err(|e| format!("Error hashing genesis state: {:?}", e))?;