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/store/src/lib.rs

@@ -271,7 +271,7 @@ mod tests {
     fn simplediskdb() {
         let dir = tempdir().unwrap();
         let path = dir.path();
-        let store = LevelDB::open(&path).unwrap();
+        let store = LevelDB::open(path).unwrap();
 
         test_impl(store);
     }