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/state-db/src/lib.rs

@@ -36,6 +36,7 @@
 #[macro_use] extern crate parity_codec_derive;
 extern crate parking_lot;
 extern crate parity_codec as codec;
+#[cfg(test)]
 extern crate substrate_primitives as primitives;
 
 mod noncanonical;