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>

account_manager/src/validator/exit.rs

@@ -51,7 +51,7 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .long(BEACON_SERVER_FLAG)
                 .value_name("NETWORK_ADDRESS")
                 .help("Address to a beacon node HTTP API")
-                .default_value(&DEFAULT_BEACON_NODE)
+                .default_value(DEFAULT_BEACON_NODE)
                 .takes_value(true),
         )
         .arg(