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/wasm/Cargo.lock

@@ -130,7 +130,6 @@ version = "0.1.0"
 dependencies = [
  "parity-codec 2.0.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_version 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
- "sr-io 0.1.0",
  "sr-std 0.1.0",
  "substrate-primitives 0.1.0",
 ]