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/srml/timestamp/src/lib.rs

@@ -39,15 +39,14 @@ extern crate sr_std as rstd;
 #[macro_use]
 extern crate srml_support as runtime_support;
 
-#[cfg(any(feature = "std", test))]
-extern crate sr_io as runtime_io;
-
 #[cfg(feature = "std")]
 #[macro_use]
 extern crate serde_derive;
 
 #[cfg(test)]
 extern crate substrate_primitives;
+#[cfg(test)]
+extern crate sr_io as runtime_io;
 extern crate sr_primitives as runtime_primitives;
 extern crate srml_system as system;
 extern crate srml_consensus as consensus;