Fix clippy warnings (#1385)

## Issue Addressed

NA

## Proposed Changes

Fixes most clippy warnings and ignores the rest of them, see issue #1388.

beacon_node/network/src/persisted_dht.rs

@@ -1,5 +1,4 @@
 use eth2_libp2p::Enr;
-use rlp;
 use std::sync::Arc;
 use store::{DBColumn, Error as StoreError, HotColdDB, ItemStore, StoreItem};
 use types::{EthSpec, Hash256};