More dependency cleanup (#825)

* Removes `rlp` dependency

* Enable warnings about unused crates in the CI

* Remove all the unused crates

* Make tests working again

substrate/core/executor/src/lib.rs

@@ -38,13 +38,9 @@ extern crate substrate_state_machine as state_machine;
 extern crate sr_version as runtime_version;
 extern crate substrate_trie as trie;
 
-extern crate serde;
 extern crate wasmi;
 extern crate byteorder;
-extern crate trie_root;
 extern crate parking_lot;
-extern crate twox_hash;
-extern crate hash_db;
 
 #[macro_use]
 extern crate log;