remove re-export of parking_lot in util (#6435)

rpc_client/src/client.rs

@@ -10,7 +10,7 @@ use std::time;
 
 use std::path::PathBuf;
 use hash::keccak;
-use util::Mutex;
+use parking_lot::Mutex;
 use url::Url;
 use std::fs::File;
 

rpc_client/src/lib.rs

@@ -6,6 +6,7 @@ extern crate futures;
 extern crate jsonrpc_core;
 extern crate jsonrpc_ws_server as ws;
 extern crate parity_rpc as rpc;
+extern crate parking_lot;
 extern crate rand;
 extern crate serde;
 extern crate serde_json;