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>

common/eth2/src/lighthouse_vc/http_client.rs

@@ -35,7 +35,7 @@ pub fn parse_pubkey(secret: &str) -> Result<PublicKey, Error> {
         &secret[SECRET_PREFIX.len()..]
     };
 
-    serde_utils::hex::decode(&secret)
+    serde_utils::hex::decode(secret)
         .map_err(|e| Error::InvalidSecret(format!("invalid hex: {:?}", e)))
         .and_then(|bytes| {
             if bytes.len() != PK_LEN {