fix: remove dead code

fendermint/vm/interpreter/src/fvm/observe.rs

@@ -83,14 +83,6 @@ impl_traceables!(
 
 /// Hex encoded hash.
 pub type HashHex<'a> = &'a str;
-// Hex encodable block hash.
-pub struct HexEncodableBlockHash(pub Vec<u8>);
-
-impl fmt::Debug for HexEncodableBlockHash {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", hex::encode(&self.0))
-    }
-}
 
 #[derive(Debug)]
 pub struct CheckpointCreated {