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>

crypto/eth2_key_derivation/src/derived_key.rs

@@ -127,7 +127,7 @@ fn mod_r(bytes: &[u8]) -> ZeroizeHash {
     debug_assert!(x_slice.len() <= HASH_SIZE);
 
     let mut output = ZeroizeHash::zero();
-    output.as_mut_bytes()[HASH_SIZE - x_slice.len()..].copy_from_slice(&x_slice);
+    output.as_mut_bytes()[HASH_SIZE - x_slice.len()..].copy_from_slice(x_slice);
     output
 }