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/sr-version/src/lib.rs

@@ -18,9 +18,6 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-#[cfg(feature = "std")]
-extern crate serde;
-
 #[cfg(feature = "std")]
 #[macro_use]
 extern crate serde_derive;
@@ -32,8 +29,6 @@ extern crate sr_std as rstd;
 #[macro_use]
 extern crate parity_codec_derive;
 
-extern crate parity_codec as codec;
-
 #[cfg(feature = "std")]
 use std::fmt;
 #[cfg(feature = "std")]