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>

remote_signer/backend/src/zeroize_string.rs

@@ -73,7 +73,7 @@ mod object {
     #[test]
     fn v_u8_zeroized() {
         // Create from `hex_string_to_bytes`, and record the pointer to its buffer.
-        let mut decoded_bytes = hex_string_to_bytes(&SECRET_KEY_1.to_string()).unwrap();
+        let mut decoded_bytes = hex_string_to_bytes(SECRET_KEY_1).unwrap();
         let old_pointer = decoded_bytes.as_ptr() as usize;
 
         // Do something with the borrowed vector, and zeroize.
@@ -185,17 +185,17 @@ mod functions {
         );
 
         assert_eq!(
-            hex_string_to_bytes(&SECRET_KEY_1).unwrap(),
+            hex_string_to_bytes(SECRET_KEY_1).unwrap(),
             SECRET_KEY_1_BYTES
         );
 
         assert_eq!(
-            hex_string_to_bytes(&PUBLIC_KEY_1).unwrap(),
+            hex_string_to_bytes(PUBLIC_KEY_1).unwrap(),
             PUBLIC_KEY_1_BYTES.to_vec()
         );
 
         assert_eq!(
-            hex_string_to_bytes(&SIGNING_ROOT).unwrap(),
+            hex_string_to_bytes(SIGNING_ROOT).unwrap(),
             SIGNING_ROOT_BYTES.to_vec()
         );